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

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Message-ID: <AANLkTinnuxvx9+eNvCqjK2GW2XQPn4LjK4WfMj=f20Xw@mail.gmail.com>
Date:	Mon, 10 Jan 2011 22:08:48 -0500
From:	Mike Frysinger <vapier@...too.org>
To:	linux-serial@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] drivers: char: hvc: add Blackfin JTAG console support

On Mon, Jan 10, 2011 at 21:43, Mike Frysinger wrote:
> This converts the existing bfin_jtag_comm TTY driver to the HVC layer so
> that the common HVC code can worry about all of the TTY/polling crap and
> leave the Blackfin code to worry about the Blackfin bits.

ive noticed a few downsides of converting to this ...

hvc cant be built as a module.  but not that big of a deal i guess.

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 ?

the code size also increased a bit ... old driver added up to ~1.7KiB
while hvc stuff is like ~4KiB.  but hvc does a nice job of keeping the
Blackfin-specific piece small.  so i guess it's a wash.
-mike
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