From: Peter Stuge <peter@stuge.se>
To: "Noralf Trønnes" <noralf@tronnes.org>
Cc: "open list:DRM PANEL DRIVERS" <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH 2/2] drm/gud: Use scatter-gather USB bulk transfer
Date: Tue, 15 Jun 2021 09:17:51 +0000 [thread overview]
Message-ID: <20210615091751.27367.qmail@stuge.se> (raw)
In-Reply-To: <0c688720-08d5-452a-31d1-db5020075d23@tronnes.org>
Hi Noralf,
Noralf Trønnes wrote:
> >> +static int gud_usb_bulk(struct gud_device *gdrm, size_t len)
..
> >> + timer_setup_on_stack(&ctx.timer, gud_usb_bulk_timeout, 0);
> >> + mod_timer(&ctx.timer, jiffies + msecs_to_jiffies(3000));
> >> +
> >> + usb_sg_wait(&ctx.sgr);
> >> +
> >> + if (!del_timer_sync(&ctx.timer))
> >> + ret = -ETIMEDOUT;
..
> > Mention in the commit message that sending USB bulk transfers with
> > an sglist could be unstable
Can you explain a bit about /how/ it is unstable?
As you write, usb_bulk_msg() (as used before) has a timeout which is
passed to the host controller hardware and implemented there.
I haven't used SG with kernel USB but I would expect such a timeout
to still be available with SG?
> usb_bulk_msg() has builtin timeout handling and during development of
> a microcontroller gadget implementation I've triggered this timeout
> several times when the uC usb interrupts stopped firing.
The device not responding to bulk packets scheduled and sent by the host
is a real error /in the device/ and thus not neccessarily something the
kernel must handle gracefully.. I think it's quite nice to do so, but
one can argue that it's not strictly required.
But more importantly: Remember that bulk transfer has no delivery time
guarantee. It can take indefinitely long until a bulk transfer is
scheduled by the host on a busy bus which is starved with more
important things (control, interrupt, iso transfers) - that's not
an error at all, and may be indistinguishable from the device not
responding to packets actually sent by the host.
Having a timeout is important, I just expect the USB SG interface to
support it since it is the hardware that times out in the non-SG case.
And since this is essentially real time data maybe a shorter timeout
is better? 3 seconds seems really long.
The timeout must include all latency for a frame, so e.g. 16ms (60 Hz)
is too short for sure. But maybe something like 500ms?
//Peter
next prev parent reply other threads:[~2021-06-15 9:24 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-29 18:01 [PATCH 1/2] drm/gud: Free buffers on device removal Noralf Trønnes
2021-03-29 18:01 ` [PATCH 2/2] drm/gud: Use scatter-gather USB bulk transfer Noralf Trønnes
2021-06-14 20:54 ` Linus Walleij
2021-06-15 8:48 ` Noralf Trønnes
2021-06-15 9:17 ` Peter Stuge [this message]
2021-06-15 12:19 ` Noralf Trønnes
2021-06-14 20:31 ` [PATCH 1/2] drm/gud: Free buffers on device removal Linus Walleij
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