Re: [PATCH] scsi: ufs: mark HPB support as BROKEN

Bart Van Assche
2021-10-28T20:53:40Z

On 10/28/21 1:33 PM, James Bottomley wrote:
> On Thu, 2021-10-28 at 13:21 -0700, Bart Van Assche wrote:
> [...]
>> Hi James,
>>
>> The help with trying to find a solution is appreciated.
>>
>> One of the software developers who is familiar with HPB explained to
>> me that READ BUFFER and WRITE BUFFER commands may be received in an
>> arbitrary order by UFS devices. The UFS HPB spec requires UFS devices
>> to be able to stash up to 128 such pairs. I'm concerned that leaving
>> out WRITE BUFFER commands only will break the HPB protocol in a
>> subtle way.
>
> Based on the publicly available information (the hotstorage paper) I
> don't belive it can. The Samsung guys also appear to confirm that the
> use of WRITE BUFFER is simply an optimzation for large requests:
>
> https://lore.kernel.org/all/20211025051654epcms2p36b259d237eb2b8b885210148118c5d3f@epcms2p3/
>
> As did the excerpt from the spec you posted. It will cause slowdowns
> for reads of > 32kb, because they have to go through the native FTL
> lookup now, but there shouldn't be any functional change. Unless
> there's anything else in the proprietary spec that contradicts this?

Are the UFS standards really proprietary? On https://www.t10.org/pubs.htm
I found the following: "Approved American National Standards and Technical
Reports may be purchased from: [ ... ]". And on
https://www.jedec.org/document_search?search_api_views_fulltext=hpb I found
the following: "Available for purchase: $80.00". It seems to me that SCSI
and JEDEC standards are available under similar conditions?

Regarding the question about the impact of leaving out WRITE BUFFER
commands on the HPB protocol, I hope that one of the HPB experts will be so
kind to answer that question.

Bart.