linux-kernel - Re: [PATCH 1/4] mmc: sdhci-esdhc: Add SDHCI_QUIRK_32BIT_DMA

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Message-ID: <19f1fd58-6b56-4ba5-4fdf-9cc845b55c27@wsystem.com>
Date:   Thu, 4 May 2017 11:00:50 +0200
From:   Benoît Thébaudeau <benoit@...stem.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mmc@...r.kernel.org, Ulf Hansson <ulf.hansson@...aro.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Wolfram Sang <wsa@...-dreams.de>
Subject: Re: [PATCH 1/4] mmc: sdhci-esdhc: Add SDHCI_QUIRK_32BIT_DMA_ADDR

On 04/05/2017 10:47, Arnd Bergmann wrote:
> On Wed, May 3, 2017 at 12:05 PM, Benoît Thébaudeau <benoit@...stem.com> wrote:
>> The eSDHC can only DMA from 32-bit-aligned addresses.
>>
>> This fixes the following test cases of mmc_test:
>>   11:   Badly aligned write
>>   12:   Badly aligned read
>>   13:   Badly aligned multi-block write
>>   14:   Badly aligned multi-block read
>>
>> Signed-off-by: Benoît Thébaudeau <benoit@...stem.com>
> 
> Is this the right description? I thought that SDHCI_QUIRK_32BIT_DMA_ADDR
> was for devices that cannot address high memory above 0xffffffff, rather than
> requiring a specific alignment.
> 
> If this is indeed an address range problem rather than an alignment problem,
> are you sure it is the SD controller that is wrong here, rather than having a
> 64-bit DMA capable  SDHCI connected to a 32-bit parent bus? In the
> latter case, the dma-ranges property in the parent bus should limit
> the addressing, not the device.

No, this is the right description. This quirk really is about alignment, and not
about address range. See:

drivers/mmc/host/sdhci.h:
>---
/* Controller can only DMA from 32-bit aligned addresses */
#define SDHCI_QUIRK_32BIT_DMA_ADDR			(1<<7)
<---

drivers/mmc/host/sdhci.c @ sdhci_prepare_data():
>---
		offset_mask = 0;
[...]
			if (host->quirks & SDHCI_QUIRK_32BIT_DMA_ADDR)
				offset_mask = 3;
[...]
				if (sg->offset & offset_mask) {
					DBG("Reverting to PIO because of bad alignment\n");
					host->flags &= ~SDHCI_REQ_USE_DMA;
					break;
				}
<---

Benoît