linux-kernel - Re: [PATCH v2] staging: android: ion: Allocate from heap ID directly without mask

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Message-ID: <20190215105057.jujgm4k77rhkvmo7@DESKTOP-E1NTVVP.localdomain>
Date:   Fri, 15 Feb 2019 10:50:58 +0000
From:   Brian Starkey <Brian.Starkey@....com>
To:     John Stultz <john.stultz@...aro.org>
CC:     "Andrew F. Davis" <afd@...com>, Laura Abbott <labbott@...hat.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Christoph Hellwig <hch@...radead.org>,
        Liam Mark <lmark@...eaurora.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        lkml <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>, nd <nd@....com>
Subject: Re: [PATCH v2] staging: android: ion: Allocate from heap ID directly
 without mask

Hi John,

On Thu, Feb 14, 2019 at 09:38:29AM -0800, John Stultz wrote:
> 
[snip]

> Some thoughts, as this ABI break has the potential to be pretty painful.
> 
> 1) Unfortunately, this ABI is exposed *through* libion via
> ion_alloc/ion_alloc_fd out to gralloc implementations. Which means it
> will have a wider impact to vendor userland code.

I figured libion could fairly easily loop through all the set bits in
heap_mask and call the ioctl for each until it succeeds. That
preserves the old behaviour from the libion clients' perspective.

> 
> 2) For patches that cause ABI breaks, it might be good to make it
> clear in the commit what the userland impact looks like in userspace,
> possibly with an example, so the poor folks who bisect down the change
> as breaking their system in a year or so have a clear example as to
> what they need to change in their code.
> 
> 3) Also, its not clear how a given userland should distinguish between
> the different ABIs.  We already have logic in libion to distinguish
> between pre-4.12 legacy and post-4.12 implementations (using implicit
> ion_free() behavior). I don't see any such check we can make with this
> code. Adding another ABI version may require we provide an actual
> interface version ioctl.
> 

A slightly fragile/ugly approach might be to attempt a small
allocation with a heap_mask of 0xffffffff. On an "old" implementation,
you'd expect that to succeed, whereas it would/could be made to fail
in the "new" one.

Thanks,
-Brian

> 
> thanks
> -john