Hi,
On 11/21/16, 1:11 PM, "Arnd Bergmann" <arnd@xxxxxxxx> wrote:
>On Monday, November 21, 2016 11:08:57 AM CET Linus Walleij wrote:
>> I've had it with this code now.
>>
>> The packed command support is a complex hurdle in the MMC/SD block
>> layer, around 500+ lines of code which was introduced in 2013 in
>> commits
>>
>> ce39f9d17c14 ("mmc: support packed write command for eMMC4.5 devices")
>> abd9ac144947 ("mmc: add packed command feature of eMMC4.5")
>>
>> ...and since then it has been rotting. The original author of the
>> code has disappeared from the community and the mail address is
>> bouncing.
>>
>> For the code to be exercised the host must flag that it supports
>> packed commands, so in mmc_blk_prep_packed_list() which is called for
>> every single request, the following construction appears:
>>
>> u8 max_packed_rw = 0;
>>
>> if ((rq_data_dir(cur) == WRITE) &&
>> mmc_host_packed_wr(card->host))
>> max_packed_rw = card->ext_csd.max_packed_writes;
>>
>> if (max_packed_rw == 0)
>> goto no_packed;
>>
>> This has the following logical deductions:
>>
>> - Only WRITE commands can really be packed, so the solution is
>> only half-done: we support packed WRITE but not packed READ.
>> The packed command support has not been finalized by supporting
>> reads in three years!
>
>Packed reads don't make a lot of sense, there is very little
>for an MMC to optimize in reads that it can't already do without
>the packing. For writes, packing makes could be an important
>performance optimization, if the eMMC supports it.
>
>I've added Luca Porzio and Alex Lemberg to Cc. I think they
>are subscribed to the list already, but it would be good to
>get some estimate from them too about how common the packed
>write support is on existing hardware from their respective
>employers before we kill it off.
Correct, in general there is no value in using packed for Read.
But I can’t say this for all existing flash management solution.
The eMMC spec allows to use it for Read as well.
>
>If none of Samsung/Micron/Sandisk are currently shipping
>devices that support eMMC-4.5 packed commands but don't
>support the eMMC-5.1 command queuing (which IIRC is a more
>general way to achieve the same), we probably don't need to
>worry about it too much.
Please note that even by having eMMC-5.1 device,
not all chipset vendors are having HW/SW CMDQ support.
So they might be using packed commands instead.
>
>> - mmc_host_packed_wr() is just a static inline that checks
>> host->caps2 & MMC_CAP2_PACKED_WR. The problem with this is
>> that NO upstream host sets this capability flag! No driver
>> in the kernel is using it, and we can't test it. Packed
>> command may be supported in out-of-tree code, but I doubt
>> it. I doubt that the code is even working anymore due to
>> other refactorings in the MMC block layer, who would
>> notice if patches affecting it broke packed commands?
>> No one.
>
>This is a very good indication that it's not really used.
>It would be useful to also check out the Android AOSP
>kernel tree to see if it's in there, if anything important
>supports packed commands, it's probably in there.
As far as I can say from reviewing the mobile (Android)
platforms kernel source (from different vendors),
many of them are enabling Packed Commands.
>
> Arnd
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