From: "Noralf Trønnes" <noralf@tronnes.org>
To: Peter Stuge <peter@stuge.se>
Cc: Lubomir Rintel <lkundrak@v3.sk>,
linux-usb@vger.kernel.org, sam@ravnborg.org,
dri-devel@lists.freedesktop.org, balbi@kernel.org
Subject: Re: [PATCH v3 0/6] Generic USB Display driver
Date: Thu, 16 Jul 2020 19:43:39 +0200 [thread overview]
Message-ID: <915b4e6b-2d54-800c-0bbf-099504c70c69@tronnes.org> (raw)
In-Reply-To: <20200714193841.18494.qmail@stuge.se>
Den 14.07.2020 21.38, skrev Peter Stuge:
> Noralf Trønnes wrote:
>>> In all cases, the driver on the host knows/has available how many bytes
>>> were successfully transfered.
>>
>> I was thinking about the device, that it could get out of sync. Let's
>> say the host sends a 1k framebuffer and half of it gets transferred and
>> the rest fails for some reason. Lubomir's MCU implementation has an
>> endpoint max size of 64 bytes and a callback is called for each packet.
>> If the 1k transfer fails at some point, will the device be able to
>> detect this and know that the next time the rx callback is called that
>> this is the start of a new framebuffer update?
>
> Ah! No, a device can not detect that the host intended to send more (bulk)
> packets but e.g. timed out.
>
> I can't immediately think of other reasons for a larger transfer to fail,
> which still allow communication to continue.
>
> When the host recognizes a timeout with partial data transfer it could
> simply send the remaining data in a new transfer.
>
>
> While the device can not detect host intent, the protocol could allow
> devices to specify requirements, e.g. that the host always sends full frames.
>
> In any case, please avoid making a control request mandatory for frame
> transfer.
>
> Because control requests are scheduled differently onto the wire and
> because they consist of more packets than bulk data, a control request
> will interrupt a bulk data stream and likely introduce unneccessary latency.
>
> If synchronization is always required then I'd suggest to place it inline
> with frame data, e.g. in the first packet byte.
>
> If synchronization is only required in rare cases then a control transfer
> is probably the better choice, to not waste any inline bytes.
>
> But the optimum would be that the device can describe its needs to the host
> and the host driver ensures that the device always receives the data it needs.
>
> Do you think this is possible?
>
Looking at the host driver I see that I need to fix it so that it
requeues the update if it fails (on SET_BUFFER or bulk out). Currently
it just goes back to sleep waiting for userspace to announce a new change.
I would like to avoid having a special case for retrying the failing
part of a bulk transfer for devices that only want full updates, I would
prefer to use the common error path of retrying the whole update.
This is my suggestion for the new flag:
/*
* Always send the entire framebuffer when flushing changes.
* The GUD_DRM_USB_REQ_SET_BUFFER request will not be sent before each
bulk transfer,
* it will only be sent if the previous bulk transfer had failed. This
is done to
* inform the device that the previous update failed and that a new one
is started.
*
* This flag can not be used in combination with compression.
*/
#define GUD_DRM_DISPLAY_FLAG_FULL_UPDATE BIT(1)
Noralf.
next prev parent reply other threads:[~2020-07-16 17:43 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-29 17:56 [PATCH v3 0/6] Generic USB Display driver Noralf Trønnes
2020-05-29 17:56 ` [PATCH v3 1/6] drm/client: Add drm_client_init_from_id() Noralf Trønnes
2020-05-29 17:56 ` [PATCH v3 2/6] drm/client: Add drm_client_modeset_disable() Noralf Trønnes
2020-05-29 17:56 ` [PATCH v3 3/6] drm/client: Add a way to set modeset, properties and rotation Noralf Trønnes
2020-05-29 17:56 ` [PATCH v3 4/6] drm: Add Generic USB Display driver Noralf Trønnes
2020-05-29 22:45 ` Peter Stuge
2020-06-01 16:57 ` Noralf Trønnes
2020-06-02 0:12 ` Peter Stuge
2020-06-02 2:32 ` Alan Stern
2020-06-02 5:21 ` Peter Stuge
2020-06-02 15:27 ` Alan Stern
2020-06-02 18:38 ` Peter Stuge
2020-06-05 12:03 ` Noralf Trønnes
2020-06-02 11:46 ` Noralf Trønnes
2020-07-14 15:30 ` Noralf Trønnes
2020-06-03 19:17 ` Noralf Trønnes
2020-05-29 17:56 ` [PATCH v3 5/6] drm/gud: Add functionality for the USB gadget side Noralf Trønnes
2020-05-29 17:56 ` [PATCH v3 6/6] usb: gadget: function: Add Generic USB Display support Noralf Trønnes
2020-06-02 17:05 ` Felipe Balbi
2020-06-02 19:14 ` Noralf Trønnes
2020-06-03 7:10 ` Felipe Balbi
2020-07-09 16:32 ` [PATCH v3 0/6] Generic USB Display driver Lubomir Rintel
2020-07-14 13:33 ` Noralf Trønnes
2020-07-14 17:40 ` Peter Stuge
2020-07-14 19:03 ` Noralf Trønnes
2020-07-14 19:38 ` Peter Stuge
2020-07-16 17:43 ` Noralf Trønnes [this message]
2020-07-15 7:30 ` Lubomir Rintel
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