Re: [patchv3 2/5] MMC: Use CMD23 for multiblock transfers when we can.

[Andrei Warkentin <andreiw@motorola.com>]

On Sun, Apr 17, 2011 at 12:23 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Saturday 16 April 2011, Andrei Warkentin wrote:
>> @@ -982,6 +1016,26 @@ static const struct mmc_fixup blk_fixups[] =
>>         MMC_FIXUP("SEM08G", 0x2, 0x100, add_quirk, MMC_QUIRK_INAND_CMD38),
>>         MMC_FIXUP("SEM16G", 0x2, 0x100, add_quirk, MMC_QUIRK_INAND_CMD38),
>>         MMC_FIXUP("SEM32G", 0x2, 0x100, add_quirk, MMC_QUIRK_INAND_CMD38),
>> +
>> +       /*
>> +        * Some MMC cards experience performance degradation with CMD23
>> +        * instead of CMD12-bounded multiblock transfers. For now we'll
>> +        * white list what's good:
>> +        * 1) Certain SanDisk eMMCs with the old MMCA manfid.
>> +        * 2) All new SanDisk products.
>> +        *
>> +        * N.B. This doesn't affect SD cards.
>> +        */
>> +       MMC_FIXUP(CID_NAME_ANY, CID_MANFID_ANY, CID_OEMID_ANY, add_quirk_mmc,
>> +                 MMC_QUIRK_BLK_NO_CMD23),
>> +       MMC_FIXUP("SEM04G", 0x2, 0x100, remove_quirk_mmc, MMC_QUIRK_BLK_NO_CMD23),
>> +       MMC_FIXUP("SEM08G", 0x2, 0x100, remove_quirk_mmc, MMC_QUIRK_BLK_NO_CMD23),
>> +       MMC_FIXUP("SEM16G", 0x2, 0x100, remove_quirk_mmc, MMC_QUIRK_BLK_NO_CMD23),
>> +       MMC_FIXUP("SEM32G", 0x2, 0x100, remove_quirk_mmc, MMC_QUIRK_BLK_NO_CMD23),
>> +       MMC_FIXUP("SEM02G", 0x2, 0x100, remove_quirk_mmc, MMC_QUIRK_BLK_NO_CMD23),
>> +       MMC_FIXUP(CID_NAME_ANY, 0x45, CID_OEMID_ANY,
>> +                 remove_quirk_mmc, MMC_QUIRK_BLK_NO_CMD23),
>> +
>>         END_FIXUP
>>  };
>
> Shouldn't this better be a blacklist for known bad cards?
>
> As far as I can tell, we should always use CMD23 where possible.
>

I thought about this some more. In this space (eMMCs), it's still
going to be up to the system integrator to evaluate the system and
enable/disable certain properties (such as CMD23 use) as they make
sense. What you would want to avoid, ideally, is someone compiling a
kernel for their pre-2.6.40 embedded system, not knowing anything
amount eMMCs and CMD23, and winding up with an I/O performance
regression.

So maybe this should be a blacklist for known bad cards. And the
entire support should be a "default-N" compile option for MMCs (not
SDs). That way someone who just does an "make oldconfig" will see
"CONFIG_MMC_BLK_CMD23 - I/O performance improvement for newer eMMC
cards, may cause degradation on older cards". What do you think?

A