From: David Laight <David.Laight@ACULAB.COM>
To: "'gavinli@thegavinli.com'" <gavinli@thegavinli.com>,
"gregkh@linuxfoundation.o" <gregkh@linuxfoundation.o>,
"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>
Cc: Gavin Li <git@thegavinli.com>
Subject: RE: [PATCH] usb: devio: fix mmap() on non-coherent DMA architectures
Date: Mon, 5 Aug 2019 11:37:22 +0000 [thread overview]
Message-ID: <95109056461d4721bc2d2742b18b97ee@AcuMS.aculab.com> (raw)
In-Reply-To: <20190801220134.3295-1-gavinli@thegavinli.com>
From: gavinli@thegavinli.com
> Sent: 01 August 2019 23:02
>
> On architectures that are not (or are optionally) DMA coherent,
> dma_alloc_coherent() returns an address into the vmalloc space,
> and calling virt_to_phys() on this address returns an unusable
> physical address.
So? what is the code trying to use the return value of virt_to_phys() for?
The 'cpu physical address' isn't (usually) a very interesting number.
The value you normally want is the address the hardware should use
in order to access the memory - this isn't (in general) the same value.
(It might be different for different devices.)
ISTR that dma_alloc_coherent() returns this value to the caller.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2019-08-05 11:37 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-01 22:01 [PATCH] usb: devio: fix mmap() on non-coherent DMA architectures gavinli
2019-08-05 11:37 ` David Laight [this message]
2019-08-05 18:33 ` Gavin Li
-- strict thread matches above, loose matches on Subject: below --
2019-08-01 22:04 gavinli
2019-08-02 12:14 ` Greg KH
2019-08-02 17:57 ` Gavin Li
2019-08-05 15:17 ` Greg KH
2019-09-04 7:05 ` Greg KH
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