linux-kernel - Re: [PATCH v2 7/9] xen/gntdev: Implement dma-buf export functionality

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Date:   Thu, 7 Jun 2018 18:30:06 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Oleksandr Andrushchenko <andr2000@...il.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        jgross@...e.com, konrad.wilk@...cle.com
Cc:     daniel.vetter@...el.com, dongwon.kim@...el.com,
        matthew.d.roper@...el.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH v2 7/9] xen/gntdev: Implement dma-buf export functionality

On 06/07/2018 04:44 AM, Oleksandr Andrushchenko wrote:
> On 06/07/2018 12:48 AM, Boris Ostrovsky wrote:
>> On 06/06/2018 08:10 AM, Oleksandr Andrushchenko wrote:
>>> On 06/05/2018 01:07 AM, Boris Ostrovsky wrote:
>>>> On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:
>>
>>>> +
>>>> +static struct sg_table *
>>>> +dmabuf_exp_ops_map_dma_buf(struct dma_buf_attachment *attach,
>>>> +               enum dma_data_direction dir)
>>>> +{
>>>> +    struct gntdev_dmabuf_attachment *gntdev_dmabuf_attach =
>>>> attach->priv;
>>>> +    struct gntdev_dmabuf *gntdev_dmabuf = attach->dmabuf->priv;
>>>> +    struct sg_table *sgt;
>>>> +
>>>> +    pr_debug("Mapping %d pages for dev %p\n",
>>>> gntdev_dmabuf->nr_pages,
>>>> +         attach->dev);
>>>> +
>>>> +    if (WARN_ON(dir == DMA_NONE || !gntdev_dmabuf_attach))
>>>>
>>>> WARN_ON_ONCE. Here and elsewhere.
>>> Why? The UAPI may be used by different applications, thus we might
>>> lose warnings for some of them. Having WARN_ON will show problems
>>> for multiple users, not for the first one.
>>> Does this make sense to still use WARN_ON?
>>
>> Just as with pr_err call somewhere else the concern here is that
>> userland (which I think is where this is eventually called from?) may
>> intentionally trigger the error, flooding the log.
>>
>> And even this is not directly called from userland there is still a
>> possibility of triggering this error multiple times.
> Ok, will use WARN_ON_ONCE


In fact, is there a reason to use WARN at all? Does this condition
indicate some sort of internal inconsistency/error?

-boris