linux-kernel - Re: [PATCH] drivers/isdn/gigaset: new M101 driver

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Date:	Mon, 05 Feb 2007 02:42:09 +0100
From:	Tilman Schmidt <tilman@...p.cc>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Karsten Keil <kkeil@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	i4ldeveloper@...tserv.isdn4linux.de, linux-serial@...r.kernel.org,
	Hansjoerg Lipp <hjlipp@....de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH] drivers/isdn/gigaset: new M101 driver

Am 04.02.2007 02:56 schrieb Andrew Morton:
> On Sun, 04 Feb 2007 02:32:41 +0100 Tilman Schmidt <tilman@...p.cc> wrote:
> 
>>>> +	spin_lock_irqsave(&cs->cmdlock, flags);
>>>> +	cb = cs->cmdbuf;
>>>> +	spin_unlock_irqrestore(&cs->cmdlock, flags);
>>> It is doubtful if the locking here does anything useful.
>> It assures atomicity when reading the cs->cmdbuf pointer.
> 
> I think it's bogus.  If the quantity being copied here is more than 32-bits
> then yes, a lock is appropriate.  But if it's a single word then it's
> unlikely that the locking does anything useful.  Or there might be a bug
> here.

It's a pointer. Are reads and writes of pointer sized objects
guaranteed to be atomic on every platform? If so, I'll happily
omit the locking.

>>>> +	spin_lock_irqsave(&cs->cmdlock, flags);
>>>> +	cb->prev = cs->lastcmdbuf;
>>>> +	if (cs->lastcmdbuf)
>>>> +		cs->lastcmdbuf->next = cb;
>>>> +	else {
>>>> +		cs->cmdbuf = cb;
>>>> +		cs->curlen = len;
>>>> +	}
>>>> +	cs->cmdbytes += len;
>>>> +	cs->lastcmdbuf = cb;
>>>> +	spin_unlock_irqrestore(&cs->cmdlock, flags);
>>> Would the use of list_heads simplify things here?
>> I don't think so. The operations in list.h do not keep track of
>> the total byte count, and adding that in a race-free way appears
>> non-trivial.
> 
> Maintaining a byte count isn't related to maintaining a list.

Sure. But your question was whether the list.h operations would
simplify this code. AFAICS it wouldn't, because the necessity
of maintaining the byte count would complicate a list.h based
solution beyond the current one. Also, this is part of the
interface with the components of the Gigaset driver which are
already part of the kernel. Changing this to a list_head now
would require significant changes in those other parts, too.

>>>> +	tail = atomic_read(&inbuf->tail);
>>>> +	head = atomic_read(&inbuf->head);
>>>> +	gig_dbg(DEBUG_INTR, "buffer state: %u -> %u, receive %u bytes",
>>>> +		head, tail, count);
>>>> +
>>>> +	if (head <= tail) {
>>>> +		n = RBUFSIZE - tail;
>>>> +		if (count >= n) {
>>>> +			/* buffer wraparound */
>>>> +			memcpy(inbuf->data + tail, buf, n);
>>>> +			tail = 0;
>>>> +			buf += n;
>>>> +			count -= n;
>>>> +		} else {
>>>> +			memcpy(inbuf->data + tail, buf, count);
>>>> +			tail += count;
>>>> +			buf += count;
>>>> +			count = 0;
>>>> +		}
>>>> +	}
>>> Perhaps the (fairly revolting) circ_buf.h can be used for this stuff.
>> It probably could, but IMHO readability would suffer rather than improve.
> 
> How about kernel/kfifo.c?

That would indeed fit the bill. But again, this code matches
parts of drivers/isdn/gigaset which are already in the kernel,
and changing it here would require significant corresponding
changes in those other parts.

I'll gladly consider your last two propositions (list_head for
cs->lastcmdbuf and kfifo for cs->inbuf) for a future revision of
the entire set of drivers in drivers/isdn/gigaset, but it goes
way beyond the scope of the present patch, which merely aims at
adding the missing M101 hardware driver.

Thanks,
Tilman

-- 
Tilman Schmidt                          E-Mail: tilman@...p.cc
Bonn, Germany
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