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

[Sumit Semwal]

Hi Russell!

On 29 January 2015 at 20:09, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Tue, Jan 27, 2015 at 01:55:54PM +0530, Sumit Semwal wrote:
>> +/*
>> + * recalc_constraints - recalculates constraints for all attached devices;
>> + *  useful for detach() recalculation, and for dma_buf_recalc_constraints()
>> + *  helper.
>> + *  Returns recalculated constraints in recalc_cons, or error in the unlikely
>> + *  case when constraints of attached devices might have changed.
>> + */
>
Thanks for your valuable review comments!

> Please see kerneldoc documentation for the proper format of these comments.
These are static functions, and as such kerneldoc doesn't enforce
kernel-doc style comments, so in the dma-buf files, we've not followed
them for static functions.
That said, it is certainly a valuable advice, and I could create a
separate patch-set for updating the documentation for the static
functions as well.
>
>> +static int recalc_constraints(struct dma_buf *dmabuf,
>> +                           struct device_dma_parameters *recalc_cons)
>> +{
>> +     struct device_dma_parameters calc_cons;
>> +     struct dma_buf_attachment *attach;
>> +     int ret = 0;
>> +
>> +     init_constraints(&calc_cons);
>> +
>> +     list_for_each_entry(attach, &dmabuf->attachments, node) {
>> +             ret = calc_constraints(attach->dev, &calc_cons);
>> +             if (ret)
>> +                     return ret;
>> +     }
>> +     *recalc_cons = calc_cons;
>> +     return 0;
>> +}
>> +
>>  /**
>>   * dma_buf_export_named - Creates a new dma_buf, and associates an anon file
>>   * with this buffer, so it can be exported.
>> @@ -313,6 +373,9 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops,
>>       dmabuf->ops = ops;
>>       dmabuf->size = size;
>>       dmabuf->exp_name = exp_name;
>> +
>> +     init_constraints(&dmabuf->constraints);
>> +
>>       init_waitqueue_head(&dmabuf->poll);
>>       dmabuf->cb_excl.poll = dmabuf->cb_shared.poll = &dmabuf->poll;
>>       dmabuf->cb_excl.active = dmabuf->cb_shared.active = 0;
>> @@ -422,7 +485,7 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>>                                         struct device *dev)
>>  {
>>       struct dma_buf_attachment *attach;
>> -     int ret;
>> +     int ret = 0;
>>
>>       if (WARN_ON(!dmabuf || !dev))
>>               return ERR_PTR(-EINVAL);
>> @@ -436,6 +499,9 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>>
>>       mutex_lock(&dmabuf->lock);
>>
>> +     if (calc_constraints(dev, &dmabuf->constraints))
>> +             goto err_constraints;
>> +
>>       if (dmabuf->ops->attach) {
>>               ret = dmabuf->ops->attach(dmabuf, dev, attach);
>>               if (ret)
>> @@ -448,6 +514,7 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>>
>>  err_attach:
>>       kfree(attach);
>> +err_constraints:
>>       mutex_unlock(&dmabuf->lock);
>>       return ERR_PTR(ret);
>>  }
>> @@ -470,6 +537,8 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>>       if (dmabuf->ops->detach)
>>               dmabuf->ops->detach(dmabuf, attach);
>>
>> +     recalc_constraints(dmabuf, &dmabuf->constraints);
>> +
>
> To me, this whole thing seems horribly racy.
>
> What happens if subsystem X creates a dmabuf, which is passed to
> userspace. It's then passed to subsystem Y, which starts making use
> of it, calling dma_buf_map_attachment() on it.
>
> The same buffer is also passed (via unix domain sockets) to another
> program, which then passes it independently into subsystem Z, and
> subsystem Z has more restrictive DMA constraints.
>
> What happens at this point?
>
> Subsystems such as DRM cache the scatter table, and return it for
> subsequent attach calls, so DRM drivers using the default
> drm_gem_map_dma_buf() implementation would not see the restrictions
> placed upon the dmabuf.  Moreover, the returned scatterlist would not
> be modified for those restrictions either.
>
> What would other subsystems do?
>
> This needs more thought before it's merged.
>
> For example, in the above situation, should we deny the ability to
> create a new attachment when a dmabuf has already been mapped by an
> existing attachment?  Should we deny it only when the new attachment
> has more restrictive DMA constraints?
>
So, short answer is, it is left to the exporter to decide. The dma-buf
framework should not even attempt to decide or enforce any of the
above.

At each dma_buf_attach(), there's a callback to the exporter, where
the exporter can decide, if it intends to handle these kind of cases,
on the best way forward.

The exporter might, for example, decide to migrate backing storage,
should there be a need to do so, or simply deny when the new
attachment has more restrictive DMA constraints, as you mentioned as a
possibility.

These changes simply allow the exporter, should it wish to, to take
the DMA constraints into consideration while making those decisions.
For the current cases, it should not even matter if the DMA
constraints aren't shared by the devices.

> Please consider the possible sequences of use (such as the scenario
> above) when creating or augmenting an API.
>

I tried to think of the scenarios I could think of, but If you still
feel this approach doesn't help with your concerns, I'll graciously
accept advice to improve it.

Once again, thanks for reviewing these changes!

> --
> FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
> according to speedtest.net.


Best regards,
~Sumit.