From: Arnd Bergmann <arnd@arndb.de>
To: Li Chen <me@linux.beauty>
Cc: Arnd Bergmann <arnd@arndb.de>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: Why dma_alloc_coherent don't return direct mapped vaddr?
Date: Fri, 22 Jul 2022 08:50:17 +0200 [thread overview]
Message-ID: <CAK8P3a2Nzky44cj4GN910c0DK=gAhR5keFSOsmwHaBkELPCStQ@mail.gmail.com> (raw)
In-Reply-To: <18223d6f513.b214956e3076389.5726369280729584331@linux.beauty>
On Fri, Jul 22, 2022 at 4:57 AM Li Chen <me@linux.beauty> wrote:
> ---- On Thu, 21 Jul 2022 15:06:57 +0800 Arnd Bergmann <arnd@arndb.de> wrote ---
> > in between.
>
> Thanks for your answer! My device is a misc character device, just like
> https://lwn.net/ml/linux-kernel/20220711122459.13773-5-me@linux.beauty/
> IIUC, its dma_addr is always the same with phy addr. If I want to alloc from
> reserved memory and then mmap to userspace with vm_insert_pages, are
> cma_alloc/dma_alloc_contigous/dma_alloc_from_contigous better choices?
In the driver, you should only ever use dma_alloc_coherent() for getting
a coherent DMA buffer, the other functions are just the implementation
details behind that.
To map this buffer to user space, your mmap() function should call
dma_mmap_coherent(), which in turn does the correct translation
from device specific dma_addr_t values into pages and uses the
correct caching attributes.
Arnd
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next prev parent reply other threads:[~2022-07-22 6:52 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-21 3:28 Why dma_alloc_coherent don't return direct mapped vaddr? Li Chen
2022-07-21 7:06 ` Arnd Bergmann
2022-07-22 2:57 ` Li Chen
2022-07-22 6:50 ` Arnd Bergmann [this message]
2022-07-22 8:19 ` Li Chen
2022-07-22 9:06 ` Arnd Bergmann
2022-07-22 10:31 ` Li Chen
2022-07-22 11:06 ` Arnd Bergmann
2022-07-25 2:50 ` Li Chen
2022-07-25 7:03 ` Arnd Bergmann
2022-07-25 11:06 ` Li Chen
2022-07-25 11:45 ` Arnd Bergmann
2022-07-26 6:50 ` Li Chen
2022-07-27 3:02 ` Li Chen
2022-07-27 6:40 ` Arnd Bergmann
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