linux-kernel - Re: [RFC][PATCH 1/5 v2] dma-buf: Add dma-buf heaps framework

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Message-ID: <CALAqxLUoXF-M1bGtDPLPg5qJ++e9gpjpnXrJ1_vHg1uLhWQDmA@mail.gmail.com>
Date:   Thu, 21 Mar 2019 14:16:34 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Brian Starkey <Brian.Starkey@....com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        "Andrew F. Davis" <afd@...com>, Laura Abbott <labbott@...hat.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Chenbo Feng <fengc@...gle.com>,
        Alistair Strachan <astrachan@...gle.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        nd <nd@....com>
Subject: Re: [RFC][PATCH 1/5 v2] dma-buf: Add dma-buf heaps framework

On Tue, Mar 19, 2019 at 5:08 AM Brian Starkey <Brian.Starkey@....com> wrote:
>
> Hi John,
>
> On Tue, Mar 05, 2019 at 12:54:29PM -0800, John Stultz wrote:
> > From: "Andrew F. Davis" <afd@...com>
>
> [snip]
>
> > +
> > +#define NUM_HEAP_MINORS 128
> > +static DEFINE_IDR(dma_heap_idr);
> > +static DEFINE_MUTEX(minor_lock); /* Protect idr accesses */
>
> I saw that Matthew Wilcox is trying to nuke idr:
> https://patchwork.freedesktop.org/series/57073/
>
> Perhaps a different data structure could be considered? (I don't have
> an informed opinion on which).

Thanks for pointing this out! I've just switched to using the Xarray
implementation in my tree.

> > +static int dma_heap_buffer_alloc(struct dma_heap *heap, size_t len,
> > +                              unsigned int flags)
> > +{
> > +     len = PAGE_ALIGN(len);
> > +     if (!len)
> > +             return -EINVAL;
>
> I think aligning len to pages only makes sense if heaps are going to
> allocate aligned to pages too. Perhaps that's an implicit assumption?
> If so, lets document it.

I've added a comment as such (or do you have more thoughts on where it
should be documented?), and for consistency removed the PAGE_ALIGN
usage in the heap allocator hooks.

> Why not let the heaps take care of aligning len however they want
> though?

As Andrew already said, It seems page granularity would have to be the
finest allocation granularity for dmabufs.  If heaps want to implement
their own larger granularity alignment, I don't see any reason they
would be limited there.

And for me, its mostly because I stubbed my toe implementing the heap
code w/ the first patch that didn't have the page alignment in the
generic code. :)

> > +     /* Create device */
> > +     heap->heap_devt = MKDEV(MAJOR(dma_heap_devt), heap->minor);
> > +     dev_ret = device_create(dma_heap_class,
> > +                             NULL,
> > +                             heap->heap_devt,
> > +                             NULL,
> > +                             heap->name);
> > +     if (IS_ERR(dev_ret)) {
> > +             pr_err("dma_heap: Unable to create char device\n");
> > +             return PTR_ERR(dev_ret);
> > +     }
> > +
> > +     /* Add device */
> > +     cdev_init(&heap->heap_cdev, &dma_heap_fops);
> > +     ret = cdev_add(&heap->heap_cdev, dma_heap_devt, NUM_HEAP_MINORS);
>
> Shouldn't this be s/dma_heap_devt/heap->heap_devt/ and a count of 1?
>
> Also would it be better to have cdev_add/device_create the other way
> around? First create the char device, then once it's all set up
> register it with sysfs.

Thanks for catching that! Much appreciated! Reworked as suggested.

Though I realized last week I have not figured out a consistent way to
have the heaps show up in /dev/dma_heaps/<device> on both Android and
classic Linux environments.  I need to go stare at the /dev/input/
setup code some more.

> > +     if (ret < 0) {
> > +             device_destroy(dma_heap_class, heap->heap_devt);
> > +             pr_err("dma_heap: Unable to add char device\n");
> > +             return ret;
> > +     }
> > +
> > +     return 0;
> > +}
> > +EXPORT_SYMBOL(dma_heap_add);
>
> Until we've figured out how modules are going to work, I still think
> it would be a good idea to not export this.

Done!

thanks
-john