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Subject: Re: [PATCH v1 1/2] drm/bridge/synopsys: dsi: Fix
 dsi_host_transfer() return value
To:     Brian Norris <briannorris@chromium.org>,
        Philippe CORNU <philippe.cornu@st.com>
Cc:     Bhumika Goyal <bhumirks@gmail.com>,
        Alexandre TORGUE <alexandre.torgue@st.com>,
        David Airlie <airlied@linux.ie>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        Yannick FERTRE <yannick.fertre@st.com>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@lists.infradead.org>,
        Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Ludovic BARRE <ludovic.barre@st.com>,
        Mickael REULIER <mickael.reulier@st.com>,
        Vincent ABRIOU <vincent.abriou@st.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        Thierry Reding <thierry.reding@gmail.com>
From:   Andrzej Hajda <a.hajda@samsung.com>
Message-id: <61a3e22a-811b-53e5-f735-534dfd9f67da@samsung.com>
Date:   Thu, 25 Jan 2018 13:16:10 +0100
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On 24.01.2018 19:37, Brian Norris wrote:
> Hi Philippe,
>
> On Wed, Jan 24, 2018 at 01:33:54PM +0000, Philippe CORNU wrote:
>> On 01/23/2018 10:38 PM, Brian Norris wrote:
>>> Hi Philippe,
>>>
>>> On Tue, Jan 23, 2018 at 6:26 AM, Philippe Cornu <philippe.cornu@st.com> wrote:
>>>> The dw_mipi_dsi_host_transfer() must return the number of
>>>> bytes transmitted/received on success instead of 0.
>>> I'm a little confused. As of the latest drm-misc-next I'm looking at,
>>> this still has conflicting documentation.
>>>
>>> For ->transfer():
>>>
>>> On success it shall return the number of bytes
>>>   * transmitted for write packets or the number of bytes received for read
>>>   * packets.
>>>
>>> While mipi_dsi_generic_read() says:
>>>
>>>   * Return: The number of bytes successfully read or a negative error code on
>>>   * failure.
>>>
>>> But it just returns the value that ->transfer() returns.
>>>
>> Not sure to follow you here: mipi_dsi_generic_read() will trig a dsi 
>> generic read so it has to return "the number of bytes received for read 
>> packets" as explained for the ->transfer() function... so it looks 
>> "coherent"...
>>
>> But maybe you want to point out something different?
> Actually, reading back what I wrote, I'm not sure it made sense. I think
> *I* was confusing "supporting TX only" with "supporting TX and RX". I
> believe the documentation isn't conflicting, but your current patch is a
> little misleading.
>
> With your current patch, you're returning the 'mipi_dsi_packet::size',
> which is the sum of both TX and RX.

I did not found docs saying mipi_dsi_packet::size is a sum of tx and rx.
tx and rx packets are two different packets, so they do not sum up.
But thanks for bringing it up, it shows docs are incomplete/misleading.


>  Since we only support TX right now,
> I suppose that actually is fine (because 'rx_len == 0'). But if we start
> supporting RX too, then this field is not the right one to return.
>
> Anyway, maybe this patch was fine as it was. But when you get RX
> support, this will have to be something like:
>
> 	if (msg->rx_len)
> 		return msg->rx_len;
> 	else
> 		return packet.size;
>
> BTW, does anyone actually care about seeing the number of TX bytes
> returned? That seems weird, because I wouldn't expect you'd get a good
> result from a partial TX (dunno about partial RX). And I also see that
> there are other drivers that get this all wrong too. See
> mtk_dsi_host_transfer(), which only returns 0 for TX and 'recv_cnt' for
> RX.

As far as I remember MIPI DSI standard does not allow partial TX, it is
all-or-nothing operation.

>
> So all-in-all, maybe my problem isn't that the documentation is
> conflicting, exactly, but that the requirements are somewhat odd, such
> that either implementations get it wrong (2 of 3 that I've looked at!),
> or they have to write somewhat odd special-casing.

mipi_dsi_host_ops::transfer in case of write sends only tx packet, in
case of read it sends tx packets and receives rx packet, so it
can be confusing what it should return in case of read.
IMO changing mipi_dsi_host_ops::transfer to always return number of
bytes RECEIVED or error should make it clearer and simpler.

+CC Thierry

Regards
Andrzej


>
>>> So I'm not sure whether the documentation is still wrong, or if the
>>> implementation is.
>>>
>>> Anyway, I guess maybe that isn't super-critical to *this* patch, since
>>> we don't have RX support yet...
>>>
>> The main reason why I want to "fix" this is because I do not want to 
>> explain to our customers (writing dsi panel drivers) why we have a 
>> different returned value compare to other platforms : )
> Brian
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