Date: Tue, 13 Jan 2015 17:52:29 +0100 From: Hans Petter Selasky <hps@selasky.org> To: Ian Lepore <ian@freebsd.org> Cc: kott <kmatpral@yahoo.com>, freebsd-usb@freebsd.org Subject: Re: usb_pc_cpu_flush Message-ID: <54B54D4D.3010805@selasky.org> In-Reply-To: <1421166591.14601.195.camel@freebsd.org> References: <1419359192795-5975583.post@n5.nabble.com> <5499E734.1070507@selasky.org> <1419392511197-5975691.post@n5.nabble.com> <549A811D.3060204@selasky.org> <1419416870924-5975752.post@n5.nabble.com> <1419423740820-5975763.post@n5.nabble.com> <549AB711.8070005@selasky.org> <1419431704871-5975773.post@n5.nabble.com> <549BF430.8000207@selasky.org> <1419877515606-5976832.post@n5.nabble.com> <1421133295061-5980199.post@n5.nabble.com> <1421160576.14601.175.camel@freebsd.org> <54B53956.4090708@selasky.org> <1421163656.14601.184.camel@freebsd.org> <54B54073.6000409@selasky.org> <1421166591.14601.195.camel@freebsd.org>
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On 01/13/15 17:29, Ian Lepore wrote:
> On Tue, 2015-01-13 at 16:57 +0100, Hans Petter Selasky wrote:
>> On 01/13/15 16:40, Ian Lepore wrote:
>>> On Tue, 2015-01-13 at 16:27 +0100, Hans Petter Selasky wrote:
>>>> On 01/13/15 15:49, Ian Lepore wrote:
>>>>> On Tue, 2015-01-13 at 00:14 -0700, kott wrote:
>>>>>> Yes with cache disabled, this problem is not seen. Seems to be with a issue
>>>>>> with l2 cache.
>>>>>> Thanks kott
>>>>>
>>>>> Except that there are no known problems with l2 cache on armv7 right
>>>>> now. There are known problems with the USB driver using the busdma
>>>>> routines incorrectly, which accidentally works okay on x86 platforms but
>>>>> likely not so well on others.
>>>>>
>>>>
>>>> Hi,
>>>>
>>>> If there is a problem it is in "usb_pc_cpu_flush()" or
>>>> "usb_pc_cpu_invalidate()":
>>>>
>>>> void
>>>> usb_pc_cpu_flush(struct usb_page_cache *pc)
>>>> {
>>>> if (pc->page_offset_end == pc->page_offset_buf) {
>>>> /* nothing has been loaded into this page cache! */
>>>> return;
>>>> }
>>>> bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_PREWRITE);
>>>> }
>>>>
>>>> USB has a very simple DMA sync language, either flush or invalidate.
>>>> These are used correctly from what I can see with regard to the FreeBSD
>>>> USB specification.
>>>>
>>>> If the "usb_pc_cpu_flush()" function does not cause the CPU cache to be
>>>> written to RAM before the function returns, please let me know.
>>>>
>>>> --HPS
>>>
>>> You have an incomplete concept of how busdma sync operations work. It
>>> isn't a simple "cpu cache written to ram" operation, there are bounce
>>> buffers and other complexities involved that require that the sync
>>> operations be done at the correct time in the correct order, and the
>>> current usb driver doesn't do that. Instead it does things like
>>>
>>> bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_POSTREAD);
>>> bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_PREREAD);
>>>
>>> And that's just nonsense that will lead to problems like delivering
>>> random buffer garbage to/from a device.
>>>
>>> To the degree that USB works at all on non-x86 platforms it works by
>>> accident. Usually. Except when it doesn't.
>>>
>>> -- Ian
>>>
>>
>> Hi,
>>
>> Bounce buffers are perfectly fine with USB as long as the busdma does
>> what it is told. If there is no easy way to do a simple "cache flush" or
>> "cache invalide" or bounce buffer "flush" or bounce buffer "invalidate"
>> multiple times in a row, then busdma cannot co-exist with USB. It is not
>> because I'm stubborn, but because of the way USB DMA controllers are
>> designed.
>>
>> With USB chipsets we sometimes need to read the RAM area for a single
>> buffer multiple times to poll for updates. From what I've been told in
>> the past BUSDMA does.
>>
>> --HPS
>>
>> --HPS
>>
>> --HPS
>>
>
> And so we reach the same old impasse, where you declare that USB is
> special and doesn't have to behave like other drivers, even though it is
> in no way unique in terms of having things like concurrent shared access
> to descriptor memory, something that virtually every modern NIC has.
>
Hi,
Can you give an example of a NIC driver which you consider a good
example which use DMA both for data (not only mbufs) and the control
path and use busdma as you consider to be correct?
--HPS
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