Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support

From	Vignesh R <vigneshr@ti.com>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support
Date	2017-03-03 23:10 +0100
Message-ID	<tgAn1-3FV-45@gated-at.bofh.it> (permalink)
References	(3 earlier) <tg7l0-8hz-19@gated-at.bofh.it> <tgaiR-2ds-7@gated-at.bofh.it> <tgbHY-334-23@gated-at.bofh.it> <tgfLA-5N5-7@gated-at.bofh.it> <tguUi-8v7-19@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

>>>>   
>>> Not really, I am debugging another issue with UBIFS on DRA74 EVM (ARM
>>> cortex-a15) wherein pages allocated by vmalloc are in highmem region
>>> that are not addressable using 32 bit addresses and is backed by LPAE.
>>> So, a 32 bit DMA cannot access these buffers at all.
>>> When dma_map_sg() is called to map these pages by spi_map_buf() the
>>> physical address is just truncated to 32 bit in pfn_to_dma() (as part of
>>> dma_map_sg() call). This results in random crashes as DMA starts
>>> accessing random memory during SPI read.
>>>
>>> IMO, there may be more undiscovered caveat with using dma_map_sg() for
>>> non kmalloc'd buffers and its better that spi-nor starts handling these
>>> buffers instead of relying on spi_map_msg() and working around every
>>> time something pops up.
>>>
>> Ok, I had a closer look at the SPI framework, and it seems there's a
>> way to tell to the core that a specific transfer cannot use DMA
>> (->can_dam()). The first thing you should do is fix the spi-davinci
>> driver:
>>
>> 1/ implement ->can_dma()
>> 2/ patch davinci_spi_bufs() to take the decision to do DMA or not on a
>>    per-xfer basis and not on a per-device basis
>>

This would lead to poor perf defeating entire purpose of using DMA.

>> Then we can start thinking about how to improve perfs by using a bounce
>> buffer for large transfers, but I'm still not sure this should be done
>> at the MTD level...

If its at SPI level, then I guess each individual drivers which cannot
handle vmalloc'd buffers will have to implement bounce buffer logic.

Or SPI core can be extended in a way similar to this RFC. That is, SPI
master driver will set a flag to request SPI core to use of bounce
buffer for vmalloc'd buffers. And spi_map_buf() just uses bounce buffer
in case buf does not belong to kmalloc region based on the flag.

Mark, Cyrille, Is that what you prefer?

-- 
Regards
Vignesh

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Vignesh R <vigneshr@ti.com> - 2017-03-01 09:00 +0100
  Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Cyrille Pitchen <cyrille.pitchen@atmel.com> - 2017-03-01 12:10 +0100
    Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Frode Isaksen <fisaksen@baylibre.com> - 2017-03-01 12:50 +0100
    Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Boris Brezillon <boris.brezillon@free-electrons.com> - 2017-03-01 13:40 +0100
    Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Boris Brezillon <boris.brezillon@free-electrons.com> - 2017-03-01 15:30 +0100
      Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Cyrille Pitchen <cyrille.pitchen@atmel.com> - 2017-03-01 15:40 +0100
    Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Cyrille Pitchen <cyrille.pitchen@atmel.com> - 2017-03-01 15:30 +0100
      Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Mark Brown <broonie@kernel.org> - 2017-03-01 17:00 +0100
    Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Boris Brezillon <boris.brezillon@free-electrons.com> - 2017-03-01 17:20 +0100
    Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Boris Brezillon <boris.brezillon@free-electrons.com> - 2017-03-01 18:00 +0100
      Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Frode Isaksen <fisaksen@baylibre.com> - 2017-03-02 10:10 +0100
        Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Boris Brezillon <boris.brezillon@free-electrons.com> - 2017-03-02 16:00 +0100
          Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Frode Isaksen <fisaksen@baylibre.com> - 2017-03-02 16:20 +0100
            Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Boris Brezillon <boris.brezillon@free-electrons.com> - 2017-03-02 17:00 +0100
              Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Frode Isaksen <fisaksen@baylibre.com> - 2017-03-03 10:10 +0100
          Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Cyrille Pitchen <cyrille.pitchen@atmel.com> - 2017-03-02 17:50 +0100
          Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Mark Brown <broonie@kernel.org> - 2017-03-02 19:20 +0100
            Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Boris Brezillon <boris.brezillon@free-electrons.com> - 2017-03-03 04:20 +0100
              Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Mark Brown <broonie@kernel.org> - 2017-03-03 14:20 +0100
          Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Vignesh R <vigneshr@ti.com> - 2017-03-06 12:50 +0100
        Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce  buffer support Vignesh R <vigneshr@ti.com> - 2017-03-03 23:10 +0100

csiph-web