linux-kernel - Re: [PATCH] drivers: char: hvc: add Blackfin JTAG console support

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Date:	Tue, 11 Jan 2011 00:31:57 -0500
From:	Mike Frysinger <vapier@...too.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-serial@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers: char: hvc: add Blackfin JTAG console support

On Mon, Jan 10, 2011 at 22:41, Arnd Bergmann wrote:
> On Tuesday 11 January 2011, Mike Frysinger wrote:
>> throughput seems to be a bit lower.  the Blackfin JTAG hardware has
>> basically a 4 byte fifo that is filled/consumed in a single shot.  and
>> the Blackfin can produce much faster than the host can consume.  so
>> atm, i have it send out 4 bytes, and the hvc layers take care of
>> calling back into me at some point.  if i add a busy loop to run a few
>> hundred milliseconds (like HZ/4), it runs much nicer.  but obviously
>> doesnt give anyone else time to run.  i cant schedule or anything as
>> the write layers are called with a spin_lock_irqsave.  any tips for
>> how to speed this up a bit ?  or is it a wash with hvc ?
>
> You could try lowering the hvc MIN_TIMEOUT value. If you want to
> change it for blackfin, you could probably make it a configuration
> option. However, if it really takes a few milliseconds that you need
> to wait between two accesses, MIN_TIMEOUT should already be small
> enough.
>
> You should really not need to sleep in your output function, as
> the khvcd() timeout logic tries handling this in the best way.
> There may be a bug in that logic though (wouldn't be the first one
> there), so try to see what the timeouts are when you get into
> this problem.

np, i'll poke around a bit.  i'm ok with merging this driver now since
it seems to be functionality equivalent to the old code.
-mike
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