Re: [PATCH] mmc: block: enable cache-flushing when mmc cache is on

[Michael Wu <michael@allwinnertech.com>]

On 14/03/2022 17:37, Avri Altman wrote:
>> On 14/03/2022 14:54, Adrian Hunter wrote:
>>> On 12/03/2022 06:43, Michael Wu wrote:
>>>> The mmc core enable cache on default. But it only enables
>>>> cache-flushing when host supports cmd23 and eMMC supports reliable
>> write.
>>>> For hosts which do not support cmd23 or eMMCs which do not support
>>>> reliable write, the cache can not be flushed by `sync` command.
>>>> This may leads to cache data lost.
>>>> This patch enables cache-flushing as long as cache is enabled, no
>>>> matter host supports cmd23 and/or eMMC supports reliable write or not.
>>>>
>>>
>>> Fixes tag?
>>>
>>
>> Hi Adrian,
>> My patch intend to fix the cache problem brought by the following two
>> patches:
>>
>> Fixes: d0c97cfb81ebc ("mmc: core: Use CMD23 for multiblock transfers when
>> we can.")
>> Fixes: e9d5c746246c8 ("mmc/block: switch to using blk_queue_write_cache()")
>>
>> I'm not sure if this is what you referred to ("Fixes tag"). Please correct me if I
>> misunderstood.
>>
>>>> Signed-off-by: Michael Wu <michael@allwinnertech.com>
>>>> ---
>>>>    drivers/mmc/core/block.c | 20 ++++++++++++++------
>>>>    1 file changed, 14 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
>>>> index 689eb9afeeed..1e508c079c1e 100644
>>>> --- a/drivers/mmc/core/block.c
>>>> +++ b/drivers/mmc/core/block.c
>>>> @@ -2279,6 +2279,8 @@ static struct mmc_blk_data
>> *mmc_blk_alloc_req(struct mmc_card *card,
>>>>       struct mmc_blk_data *md;
>>>>       int devidx, ret;
>>>>       char cap_str[10];
>>>> +    bool enable_cache = false;
>>>> +    bool enable_fua = false;
>>>>
>>>>       devidx = ida_simple_get(&mmc_blk_ida, 0, max_devices, GFP_KERNEL);
>>>>       if (devidx < 0) {
>>>> @@ -2375,12 +2377,18 @@ static struct mmc_blk_data
>> *mmc_blk_alloc_req(struct mmc_card *card,
>>>>                       md->flags |= MMC_BLK_CMD23;
>>>>       }
>>>>
>>>> -    if (mmc_card_mmc(card) &&
>>>> -        md->flags & MMC_BLK_CMD23 &&
>>>> -        ((card->ext_csd.rel_param & EXT_CSD_WR_REL_PARAM_EN) ||
>>>> -         card->ext_csd.rel_sectors)) {
>>>> -            md->flags |= MMC_BLK_REL_WR;
>>>> -            blk_queue_write_cache(md->queue.queue, true, true);
>>>> +    if (mmc_card_mmc(card)) {
>>>> +            if (md->flags & MMC_BLK_CMD23 &&
>>>> +                    ((card->ext_csd.rel_param & EXT_CSD_WR_REL_PARAM_EN)
>> ||
>>>> +                    card->ext_csd.rel_sectors)) {
>>>> +                    md->flags |= MMC_BLK_REL_WR;
>>>> +                    enable_fua = true;
>>>> +            }
>>>> +
>>>> +            if (mmc_cache_enabled(card->host))
>>>> +                    enable_cache = true;
>>>> +
>>>> +            blk_queue_write_cache(md->queue.queue, enable_cache,
>>>> + enable_fua);
>>>>       }
>>>
>>> Seems like we should inform block layer about SD card cache also
>>>
>>
>> I saw another mail by Avri Altman, which says few days will be needed to ask
>> internally. Shall I wait or make another change here on 'inform block layer
>> about SD card cache'?
> Please don't wait.
> 
> Thanks,
> Avri
> 
>>
>>>>
>>>>       string_get_size((u64)size, 512, STRING_UNITS_2,
>>
>> --
>> Best Regards,
>> Michael Wu
Hi Avril & Adrian,
Thanks for your efforts. Could we have an agreement now --

1. enabling-cache and cmd23/reliable-write should be independent;

 > On 14/03/2022 18:32, Adrian Hunter wrote:
 >> On 14/03/2022 09:26, Avri Altman wrote:
 >>> Hi,
 >>>> The mmc core enable cache on default. But it only enables 
cache-flushing
 >>>> when host supports cmd23 and eMMC supports reliable write.
 >>>> For hosts which do not support cmd23 or eMMCs which do not support
 >>>> reliable write, the cache can not be flushed by `sync` command.
 >>>> This may leads to cache data lost.
 >>>> This patch enables cache-flushing as long as cache is enabled, no
 >>>> matter host supports cmd23 and/or eMMC supports reliable write or
 >>>> not.
 >>> I looked in the spec and indeed couldn't find why enabling cache is
 >>> dependent of cmd23/reliable write.
 >>> Nor I was able to find the original commit log.
 >>
 >> Reliable write was added first, so it might have been an oversight:
 >>
 >> commit 881d1c25f765938a95def5afe39486ce39f9fc96
 >> Author: Seungwon Jeon <tgih.jun@samsung.com>
 >> Date:   Fri Oct 14 14:03:21 2011 +0900
 >>
 >>      mmc: core: Add cache control for eMMC4.5 device
 >>
 >>      This patch adds cache feature of eMMC4.5 Spec.
 >>      If device supports cache capability, host can utilize some
 >>      specific operations.
 >>
 >>      Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
 >>      Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
 >>      Signed-off-by: Chris Ball <cjb@laptop.org>

Here's what I found in the spec JESD84-B51:
 > 6.6.31 Cache
 > Caching of data shall apply only for the single block
 > read/write(CMD17/24), pre-defined multiple block
 > read/write(CMD23+CMD18/25) and open ended multiple block
 > read/write(CMD18/25+CMD12) commands and excludes any other access
 > e.g., to the register space(e.g., CMD6).
Which means with CMD18/25+CMD12 (without using CMD23), the cache can 
also be enabled. Maybe this could be an evidence of the independence 
between enabling-cache and cmd23/reliable-write?

2. We don't consider supporting SD in this change.

 > On 14/03/2022 19:10, Avri Altman wrote:
 >> Here is what our SD system guys wrote:
 >> " In SD we don’t support reliable write and this eMMC driver may not 
 >>    be utilizing the cache feature we added in SD5.0.
 >>   The method of cache flush is different between SD and eMMC."
 >>
 >> So adding SD seems to be out of scope of this change.

Is there anything else I can do about this patch? Thanks again.

-- 
Best Regards,
Michael Wu