linux-kernel - Re: [PATCH v2 2/2] mmc: sdhci-tegra: Specify valid DMA mask

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Message-ID: <1671523.8ABQgk2cvG@wuerfel>
Date:	Wed, 02 Mar 2016 12:25:31 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Alexandre Courbot <gnurou@...il.com>
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 v2 2/2] mmc: sdhci-tegra: Specify valid DMA mask

On Wednesday 02 March 2016 19:36:23 Alexandre Courbot wrote:
> On Wed, Mar 2, 2016 at 6:34 AM, Arnd Bergmann <arnd@...db.de> wrote:
> > On Tuesday 01 March 2016 13:32:44 Alexandre Courbot wrote:
> >> On T210, the sdhci controller can address more than 32 bits of address
> >> space. Failing to express this fact results in the use of bounce
> >> buffers and affects performance.
> >>
> >> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> >
> > I don't get this one. Why don't you just set the (SDHCI_USE_SDMA | SDHCI_USE_ADMA)
> > flags that are checked in the first patch?
> 
> The test is against (!(host->flags & (SDHCI_USE_SDMA |
> SDHCI_USE_ADMA))), (see the '!') so it will be true (and the DMA mask
> will be set) if both flags are *not* set (why we set the mask to 64
> bits here in that case, I don't know).
> 
> T210 is capable of SDMA, so we cannot use this condition for that
> purpose (maybe you missed the '!', in which case I understand why you
> were surprised).

Ok, I see now that this code was just setting a fake mask in case
of PIO mode, which doesn't apply here. That in turn means that
your first patch is wrong though:

For a device that is not DMA capable (neither SDMA nor ADMA
claimed to be supported), we should not call dma_set_mask_and_coherent()
because that is likely to fail as well. It's an ugly hack to
just override the mask in that case, and I'd say it requires
a comment explaining what is going on.

> >> @@ -289,6 +291,7 @@ static const struct sdhci_tegra_soc_data soc_data_tegra20 = {
> >>       .pdata = &sdhci_tegra20_pdata,
> >>       .nvquirks = NVQUIRK_FORCE_SDHCI_SPEC_200 |
> >>                   NVQUIRK_ENABLE_BLOCK_GAP_DET,
> >> +     .dma_mask = DMA_BIT_MASK(32),
> >>  };
> >
> > Can you describe what the specific bug is in these controllers? Do you mean they
> > support SDHCI_USE_SDMA or SDHCI_USE_ADMA in theory but you still have to prevent
> > them from using high addresses?
> 
> Ok, I think you probably missed the '!' then. :)

I missed the larger context of that check too, but I think I've got it now.

> >> @@ -353,6 +358,7 @@ static const struct sdhci_pltfm_data sdhci_tegra210_pdata = {
> >>
> >>  static const struct sdhci_tegra_soc_data soc_data_tegra210 = {
> >>       .pdata = &sdhci_tegra210_pdata,
> >> +     .dma_mask = DMA_BIT_MASK(34),
> >>  };
> >>
> >>  static const struct of_device_id sdhci_tegra_dt_match[] = {
> >
> > This one still completely weirds me out. What kind of odd limitation does
> > the controller have in Tegra 210?
> >
> > Are there actually any machines with more than 16GB?
> 
> It is not a limitation of the controller - I am just limiting the mask
> to the range of physical memory we can ever access on T210. My intent
> here is to overcome the default 32-bit mask, not to limit the range,
> so I could have set a 64-bit mask if not for my OCD. :P
> 
> But actually looking at how the various flags are interpreted in
> sdhci_add_host(), I see the following:
> 
>     /*
>      * It is assumed that a 64-bit capable device has set a 64-bit DMA mask
>      * and *must* do 64-bit DMA.  A driver has the opportunity to change
>      * that during the first call to ->enable_dma().  Similarly
>      * SDHCI_QUIRK2_BROKEN_64_BIT_DMA must be left to the drivers to
>      * implement.
>      */
>     if (caps[0] & SDHCI_CAN_64BIT)
>         host->flags |= SDHCI_USE_64_BIT_DMA;
> 
> Since this relies on what the hardware declares being capable of and
> is set on T210, I am very tempted to set a 64-bit dma_mask here and
> call it a day, but the above comment seems to suggest that this should
> have been done before.

Right, that sounds good, that also makes it independent of the specific
Tegra SoC, right?

> If you think this is cool though, I will just do that and in
> conjunction with patch 1 this will do the job nicely.

as mentioned above, I now have doubts about patch 1, why do you still
need this now?

	Arnd