linux-kernel - Re: [PATCH] mmc: sdhci-tegra: Set DMA mask

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Date:	Fri, 26 Feb 2016 16:24:34 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Alexandre Courbot <acourbot@...dia.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mmc: sdhci-tegra: Set DMA mask

On Thu, Feb 25, 2016 at 11:52 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> Actually even if we specify a dma-ranges on the parent DT node, the
>> DMA range will still be limited to 32 bits because of the following
>> code in of_dma_configure():
>>
>>     /*
>>      * Set default coherent_dma_mask to 32 bit.  Drivers are expected to
>>      * setup the correct supported mask.
>>      */
>>     if (!dev->coherent_dma_mask)
>>         dev->coherent_dma_mask = DMA_BIT_MASK(32);
>>
>>     /*
>>      * Set it to coherent_dma_mask by default if the architecture
>>      * code has not set it.
>>      */
>>     if (!dev->dma_mask)
>>         dev->dma_mask = &dev->coherent_dma_mask;
>>
>>     ....
>>     /* gets dma-ranges into dma_addr and size */
>>     ....
>>
>>
>>     *dev->dma_mask = min((*dev->dma_mask),
>>                  DMA_BIT_MASK(ilog2(dma_addr + size)));
>>
>> So unless the DMA mask is set on the device before of_dma_configure()
>> is called, the min() statement will choose the 32 bits mask that has
>> been previously set. So IIUC in any case, the driver will need to call
>> dma_set_mask()
>
> Yes, the driver definitely has to call dma_set_mask(), the property of
> the parent bus is used to make that fail when the bus doesn't support
> it.

And that's where things seem to stop working: the driver's probe
function is invoked by the platform bus, *after* of_dma_configure() is
called. So unless I am missing something there is no way for the
driver to set the DMA mask in such a way that of_dma_configure() can
see it and do the right thing.

In other words, most of the DMA mask logic in of_dma_configure()
doesn't seem to have any effect (and a 32 bits mask will be set), at
least on the platform bus.

>> Can I have your thoughts on this? Am I missing something?
>
> One point: I think the dma_set_mask() probably should be in the
> generic part of the sdhci driver, not the tegra specific portion.

Ok, but then how does the generic part of the driver knows which DMA
mask applies to the device?

> I also forget how this really needs to interact with swiotlb. I know
> we have discussed this a couple of times, but the result currently
> is lost to me.
> Maybe the answer was that if swiotlb or iommu are enabled, then
> dma_set_mask() should always succeed, but the mask should not actually
> be updated?