linux-kernel - Re: [PATCH v3 2/5] tty: Introduce SER_RS485_SOFTWARE read-only flag for struct serial

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <56454322.9050803@hurleysoftware.com>
Date:	Thu, 12 Nov 2015 20:55:46 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	"Matwey V. Kornilov" <matwey@....msu.ru>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	jslaby@...e.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
Subject: Re: [PATCH v3 2/5] tty: Introduce SER_RS485_SOFTWARE read-only flag
 for struct serial_rs485

On 11/12/2015 08:26 PM, Andy Shevchenko wrote:
> On Fri, Nov 13, 2015 at 3:11 AM, Peter Hurley <peter@...leysoftware.com> wrote:
>> On 11/12/2015 07:41 PM, Andy Shevchenko wrote:
>>> On Thu, Nov 12, 2015 at 10:22 PM, Peter Hurley <peter@...leysoftware.com> wrote:
>>>> On 11/12/2015 02:57 PM, One Thousand Gnomes wrote:
>>>
>>>> An illustrative (kernel-space) example is the mess that is dmaengine_pause().
>>>> Some DMA implementations provide the means to stop and restart DMA without
>>>> losing data and some DMA implementations do not. Unfortunately, some
>>>> advertise they support dmaengine_pause() but only for lossy uses like audio.
>>>> Because the api hides this, the query interface for pause support is
>>>> useless.
>>>
>>> The DMA pause() call means only pause with possibility to resume.
>>> There is a resume() call as well. Any driver which treats pause() as a
>>> complete stop is buggy driver and should be fixed.
>>
>> How about pause _without_ the possibility to resume?
>>
>> https://groups.google.com/d/msg/linux.kernel/Abe0hfGcgsw/H0se55wC558J
> 
> Briefly what I got from the thread that Russell shows similar view on
> the API, so that's why he was objecting to add pause/resume calls for
> a specific hardware.

Not quite.

That dmaengine driver (omap-dma) advertises that it supports pause() via
dma_get_slave_caps(). And if you call it with a cyclic channel it will pause.
However, if you call dmaengine_pause() with a slave channel it returns an error
*because the hardware can't actually meet the criteria for dmaengine_pause()*
which is pause()/resume() without data loss.

IOW, there is no method of determining a priori if dmaengine_pause() will
categorically fail for a given transfer type.

Regards,
Peter Hurley
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/