Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

Sumit Semwal
2015-02-02T05:53:59Z

Hi Russell,

On 30 January 2015 at 00:56, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Thu, Jan 29, 2015 at 01:52:09PM -0500, Rob Clark wrote:
>> Quite possibly for some of these edge some of cases, some of the
>> dma-buf exporters are going to need to get more clever (ie. hand off
>> different scatterlists to different clients). Although I think by far
>> the two common cases will be "I can support anything via an iommu/mmu"
>> and "I need phys contig".
>>
>> But that isn't an issue w/ dma-buf itself, so much as it is an issue
>> w/ drivers. I guess there would be more interest in fixing up drivers
>> when actual hw comes along that needs it..
>
> However, validating the attachments is the business of dma-buf. This
> is actual infrastructure, which should ensure some kind of sanity such
> as the issues I've raised.
>
> The whole "we can push it onto our users" is really on - what that
> results in is the users ignoring most of the requirements and just doing
> their own thing, which ultimately ends up with the whole thing turning
> into a disgusting mess - one which becomes very difficult to fix later.
>
> Now, if we're going to do the "more clever" thing you mention above,
> that rather negates the point of this two-part patch set, which is to
> provide the union of the DMA capabilities of all users. A union in
> that case is no longer sane as we'd be tailoring the SG lists to each
> user.
>
> If we aren't going to do the "more clever" thing, then yes, we need this
> code to calculate that union, but we _also_ need it to do sanity checking
> right from the start, and refuse conditions which ultimately break the
> ability to make use of that union - in other words, when the union of
> the DMA capabilities means that the dmabuf can't be represented.
>
> Unless we do that, we'll just end up with random drivers interpreting
> what they want from the DMA capabilities, and we'll have some drivers
> exporting (eg) scatterlists which satisfy the maximum byte size of an
> element, but ignoring the maximum number of entries or vice versa, and
> that'll most probably hide the case of "too small a union".
>
I agree, and I'll add the check for
max_segment_size * max_segment_count < dmabuf->size
and resend; will that be alright with you?

> It really doesn't make sense to do both either: that route is even more
> madness, because we'll end up with two classes of drivers - those which
> use the union approach, and those which don't.
>
> The KISS principle applies here.
>
> --
> FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
> according to speedtest.net.

Best regards,
~Sumit.