linux-kernel - Re: [GIT PULL] SCSI fixes for 6.10-rc4

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Date: Fri, 21 Jun 2024 21:48:06 -0400
From: "Martin K. Petersen" <martin.petersen@...cle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: James Bottomley <James.Bottomley@...senpartnership.com>,
        Andrew Morton
 <akpm@...ux-foundation.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Bart Van Assche
 <bvanassche@....org>
Subject: Re: [GIT PULL] SCSI fixes for 6.10-rc4

Linus,

> Can we place just make the rule be that new mode pages are opt-in, and
> *NOT* this kind of broken "opt-out several months later when we
> noticed that it inevitably caused problems"?

The specific problem with mode pages is that there is no way to know
whether a given page is supported without asking for it. Whereas for
most of the other things we query at discovery time, the device provides
a list of supported pages we can consult before we attempt to query the
page itself.

> Because if it isn't some mode page that we have already used for
> years, it clearly isn't *that* important.
[...]
> That should give people a big heads up that "maybe this thing isn't
> very common or commonly known about"?

It is a new feature in SCSI spearheaded by the Android folks. That's why
there isn't a lot of information available about it elsewhere.

I am super picky about having good heuristics for when we should attempt
to query a device for new protocol capabilities. In this case we lacked
a reliable indicator that the feature was supported. And since there are
non-UFS devices being implemented which support it too, restricting the
mode page query to Android/UFS devices only did not seem appropriate.

Bart: How about wrapping access to that mode page in GROUP_SUP
and RSCS? It would be nice if we could key off a VPD or two before
attempting the MODE SENSE...

-- 
Martin K. Petersen	Oracle Linux Engineering