Re: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

["Semwal, Sumit" <sumit.semwal@ti.com>]

Hi Arnd,

Thanks for your review!
On Mon, Dec 5, 2011 at 10:48 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Friday 02 December 2011, Sumit Semwal wrote:
>> This is the first step in defining a dma buffer sharing mechanism.
>
> This looks very nice, but there are a few things I don't understand yet
> and a bunch of trivial comments I have about things I spotted.
>
> Do you have prototype exporter and consumer drivers that you can post
> for clarification?
>
> In the patch 2, you have a section about migration that mentions that
> it is possible to export a buffer that can be migrated after it
> is already mapped into one user driver. How does that work when
> the physical addresses are mapped into a consumer device already?
I guess I need to clear it up in the documentation - when I said "once
all ongoing access is completed" - I meant to say "once all current
users have finished accessing and have unmapped this buffer". So I
agree with Rob - the migration would only be possible for "attached
but unmapped" buffers. I will update the documentation.
>
>> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
>> index 21cf46f..07d8095 100644
>> --- a/drivers/base/Kconfig
>> +++ b/drivers/base/Kconfig
>> @@ -174,4 +174,14 @@ config SYS_HYPERVISOR
>>
>>  source "drivers/base/regmap/Kconfig"
>>
>> +config DMA_SHARED_BUFFER
>> +     bool "Buffer framework to be shared between drivers"
>> +     default n
>> +     depends on ANON_INODES
>
> I would make this 'select ANON_INODES', like the other users of this
> feature.
Sure.

>
>> +     return dmabuf;
>> +}
>> +EXPORT_SYMBOL(dma_buf_export);
>
> I agree with Konrad, this should definitely be EXPORT_SYMBOL_GPL,
> because it's really a low-level function that I would expect
> to get used by in-kernel subsystems providing the feature to
> users and having back-end drivers, but it's not the kind of thing
> we want out-of-tree drivers to mess with.
Agreed.

>
>> +/**
>> + * dma_buf_fd - returns a file descriptor for the given dma_buf
>> + * @dmabuf:  [in]    pointer to dma_buf for which fd is required.
>> + *
>> + * On success, returns an associated 'fd'. Else, returns error.
>> + */
>> +int dma_buf_fd(struct dma_buf *dmabuf)
>> +{
>> +     int error, fd;
>> +
>> +     if (!dmabuf->file)
>> +             return -EINVAL;
>> +
>> +     error = get_unused_fd_flags(0);
>
> Why not simply get_unused_fd()?
:) oversight. Will correct.

>
>> +/**
>> + * dma_buf_attach - Add the device to dma_buf's attachments list; optionally,
>> + * calls attach() of dma_buf_ops to allow device-specific attach functionality
>> + * @dmabuf:  [in]    buffer to attach device to.
>> + * @dev:     [in]    device to be attached.
>> + *
>> + * Returns struct dma_buf_attachment * for this attachment; may return NULL.
>> + *
>
> Or may return a negative error code. It's better to be consistent here:
> either always return NULL on error, or change the allocation error to
> ERR_PTR(-ENOMEM).
Ok, that makes sense.

>
>> + */
>> +struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>> +                                             struct device *dev)
>> +{
>> +     struct dma_buf_attachment *attach;
>> +     int ret;
>> +
>> +     BUG_ON(!dmabuf || !dev);
>> +
>> +     attach = kzalloc(sizeof(struct dma_buf_attachment), GFP_KERNEL);
>> +     if (attach == NULL)
>> +             goto err_alloc;
>> +
>> +     mutex_lock(&dmabuf->lock);
>> +
>> +     attach->dev = dev;
>> +     attach->dmabuf = dmabuf;
>> +     if (dmabuf->ops->attach) {
>> +             ret = dmabuf->ops->attach(dmabuf, dev, attach);
>> +             if (!ret)
>> +                     goto err_attach;
>
> You probably mean "if (ret)" here instead of "if (!ret)", right?
yes - a stupid one! will correct.

>
>> +     /* allow allocator to take care of cache ops */
>> +     void (*sync_sg_for_cpu) (struct dma_buf *, struct device *);
>> +     void (*sync_sg_for_device)(struct dma_buf *, struct device *);
>
> I don't see how this works with multiple consumers: For the streaming
> DMA mapping, there must be exactly one owner, either the device or
> the CPU. Obviously, this rule needs to be extended when you get to
> multiple devices and multiple device drivers, plus possibly user
> mappings. Simply assigning the buffer to "the device" from one
> driver does not block other drivers from touching the buffer, and
> assigning it to "the cpu" does not stop other hardware that the
> code calling sync_sg_for_cpu is not aware of.
>
> The only way to solve this that I can think of right now is to
> mandate that the mappings are all coherent (i.e. noncachable
> on noncoherent architectures like ARM). If you do that, you no
> longer need the sync_sg_for_* calls.
I will take yours and Daniel's suggestion, and remove these; if at all
they're needed, we can add them back again later, with
/s/device/attachment as suggested by Daniel.
>
>> +#ifdef CONFIG_DMA_SHARED_BUFFER
>
> Do you have a use case for making the interface compile-time disabled?
> I had assumed that any code using it would make no sense if it's not
> available so you don't actually need this.
Ok. Though if we keep the interface compile-time disabled, the users
can actually check and fail or fall-back gracefully when the API is
not available; If I remove it, anyways the users would need to do the
same compile time check whether API is available or not, right?

>
>        Arnd
Thanks, and best regards,
~Sumit.

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