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

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Message-ID: <0c820b7d-d9e5-8a60-b9fc-86eb3ca81df8@gmail.com>
Date:   Fri, 8 Jun 2018 14:34:39 +0300
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.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 06/08/2018 01:30 AM, Boris Ostrovsky wrote:
> 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?
Well, the corresponding errors are anyways handled, so I will remove WARN
> -boris
>
>
>