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Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability

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On Tue, 1 Nov 2016, Lu Baolu wrote:
> +static int __init xdbc_init(void)
> +{
...
> +	base = ioremap_nocache(xdbc.xhci_start, xdbc.xhci_length);
> +	if (!base) {
> +		xdbc_trace("failed to remap the io address\n");
> +		ret = -ENOMEM;
> +		goto free_and_quit;
> +	}
> +
> +	early_iounmap(xdbc.xhci_base, xdbc.xhci_length);
> +	xdbc_trace("early mapped IO address released\n");
> +
> +	xdbc.xhci_base = base;
> +	offset = xhci_find_next_ext_cap(xdbc.xhci_base, 0, XHCI_EXT_CAPS_DEBUG);
> +	xdbc.xdbc_reg = (struct xdbc_regs __iomem *)(xdbc.xhci_base + offset);

This is broken. What prevents that 

     - a printk is in progress on another cpu?

     - a printk happens between the unmap and storing the new base ?

Nothing AFAICT. So this needs to be done in a safe way. And just making it

	oldbase = xdbc.xhci_base;
	base = ioremap(....);
	xdbc.xhci_base = base;
	early_iounmap(oldbase);

does not work either because the compiler can rightfully cache
xdbc.xhci_base in the write related functions. The same issue with
xdbc.xdbc_reg.

Thanks,

	tglx

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[PATCH v4 0/4] usb: early: add support for early printk through USB3 debug port Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-01 06:10 +0100
  [PATCH v4 4/4] usb: doc: add document for USB3 debug port usage Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-01 06:10 +0100
  [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-01 06:10 +0100
    Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug  capability Thomas Gleixner <tglx@linutronix.de> - 2016-11-09 10:30 +0100
      Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-10 02:20 +0100
    Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug  capability Thomas Gleixner <tglx@linutronix.de> - 2016-11-09 10:40 +0100
      Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-10 02:30 +0100
        Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug  capability Thomas Gleixner <tglx@linutronix.de> - 2016-11-10 10:00 +0100
          Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Peter Zijlstra <peterz@infradead.org> - 2016-11-10 12:50 +0100
            Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-11 05:40 +0100
              Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Peter Zijlstra <peterz@infradead.org> - 2016-11-11 13:30 +0100
          Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-11 03:30 +0100
            Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-11 03:50 +0100
  [PATCH v4 2/4] x86: add support for earlyprintk via USB3 debug port Lu Baolu <baolu.lu@linux.intel.com> - 2016-11-01 06:10 +0100

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