Re: Tearing down DMA transfer setup after DMA client has finished

From	Måns Rullgård <mans@mansr.com>
Newsgroups	linux.kernel
Subject	Re: Tearing down DMA transfer setup after DMA client has finished
Date	2016-12-08 13:30 +0100
Message-ID	<sM5ZM-65C-29@gated-at.bofh.it> (permalink)
References	(9 earlier) <sM4hj-51v-15@gated-at.bofh.it> <sM4AF-57U-21@gated-at.bofh.it> <sM4U2-5tG-11@gated-at.bofh.it> <sM5n3-5Di-7@gated-at.bofh.it> <sM5Gq-5YO-25@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

Geert Uytterhoeven <geert@linux-m68k.org> writes:

> Hi Måns,
>
> On Thu, Dec 8, 2016 at 12:47 PM, Måns Rullgård <mans@mansr.com> wrote:
>> Geert Uytterhoeven <geert@linux-m68k.org> writes:
>>> On Thu, Dec 8, 2016 at 11:54 AM, Mason <slash.tmp@free.fr> wrote:
>>>> On 08/12/2016 11:39, Vinod Koul wrote:
>>>>> On Wed, Dec 07, 2016 at 04:45:58PM +0000, Måns Rullgård wrote:
>>>>>> Vinod Koul <vinod.koul@intel.com> writes:
>>>>>>> On Tue, Dec 06, 2016 at 01:14:20PM +0000, Måns Rullgård wrote:
>>>>>>>> That's not going to work very well.  Device drivers typically request
>>>>>>>> dma channels in their probe functions or when the device is opened.
>>>>>>>> This means that reserving one of the few channels there will inevitably
>>>>>>>> make some other device fail to operate.
>>>>>>>
>>>>>>> No that doesn't make sense at all, you should get a channel only when you
>>>>>>> want to use it and not in probe!
>>>>>>
>>>>>> Tell that to just about every single driver ever written.
>>>>>
>>>>> Not really, few do yes which is wrong but not _all_ do that.
>>>>
>>>> Vinod,
>>>>
>>>> Could you explain something to me in layman's terms?
>>>>
>>>> I have a NAND Flash Controller driver that depends on the
>>>> DMA driver under discussion.
>>>>
>>>> Suppose I move the dma_request_chan() call from the driver's
>>>> probe function, to the actual DMA transfer function.
>>>>
>>>> I would want dma_request_chan() to put the calling thread
>>>> to sleep until a channel becomes available (possibly with
>>>> a timeout value).
>>>>
>>>> But Maxime told me dma_request_chan() will just return
>>>> -EBUSY if no channels are available.
>>>>
>>>> Am I supposed to busy wait in my driver's DMA function
>>>> until a channel becomes available?
>>>
>>> Can you fall back to PIO if requesting a channel fails?
>>>
>>> Alternatively, dma_request_chan() could always succeed, and
>>> dmaengine_prep_slave_sg() could fail if the channel is currently not
>>> available due to a limitation on the number of active channels, and
>>> the driver could fall back to PIO for that transfer.
>>
>> Why are we debating this nonsense?  There is an easy fix that doesn't
>> require changing the semantics of existing functions or falling back to
>> slow pio.
>
> You still want to fall back to PIO if the DMA engine is not available at all
> (e.g. DMA engine driver not compiled in, or module not loaded).

That's a choice for each device driver to make.  Some devices don't have
a pio mode at all.

-- 
Måns Rullgård

Thread

Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-11-25 13:50 +0100
  Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-11-25 14:20 +0100
    Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-11-25 15:30 +0100
      Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-11-25 15:40 +0100
        Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-11-25 16:50 +0100
  Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-11-29 19:30 +0100
    Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-06 06:10 +0100
      Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-06 13:50 +0100
        Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-06 14:20 +0100
          Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-06 16:30 +0100
            Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-06 16:40 +0100
              Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-07 00:00 +0100
          Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-07 17:40 +0100
            Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-07 17:50 +0100
              Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-08 11:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-08 12:00 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Geert Uytterhoeven <geert@linux-m68k.org> - 2016-12-08 12:20 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 12:50 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Geert Uytterhoeven <geert@linux-m68k.org> - 2016-12-08 13:10 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-08 13:20 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 13:30 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-08 17:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 17:50 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-09 08:10 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Sebastian Frias <sf84@laposte.net> - 2016-12-09 11:30 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-09 12:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished 1Måns Rullgård <mans@mansr.com> - 2016-12-09 12:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-09 18:20 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-09 18:30 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-09 19:00 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-09 18:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-09 19:00 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-09 19:20 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-09 19:30 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 12:50 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Geert Uytterhoeven <geert@linux-m68k.org> - 2016-12-08 13:10 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 13:30 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Geert Uytterhoeven <geert@linux-m68k.org> - 2016-12-08 13:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 13:50 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-08 14:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 14:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-08 13:50 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-08 16:50 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 17:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-08 16:40 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Mason <slash.tmp@free.fr> - 2016-12-08 16:50 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-08 17:30 +0100
                Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 17:50 +0100
        Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-07 17:40 +0100
          Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-07 17:50 +0100
            Re: Tearing down DMA transfer setup after DMA client has finished Vinod Koul <vinod.koul@intel.com> - 2016-12-08 11:30 +0100
              Re: Tearing down DMA transfer setup after DMA client has finished Måns Rullgård <mans@mansr.com> - 2016-12-08 12:50 +0100

csiph-web