From: Alexey Kardashevskiy <aik@ozlabs.ru>
To: Christoph Hellwig <hch@lst.de>
Cc: "Oliver O'Halloran" <oohall@gmail.com>,
linuxppc-dev@lists.ozlabs.org, "Cédric Le Goater" <clg@kaod.org>
Subject: Re: [PATCH kernel] powerpc/dma: Fix dma_map_ops::get_required_mask
Date: Tue, 8 Sep 2020 22:06:56 +1000 [thread overview]
Message-ID: <94353228-2262-cfa1-7177-7eed2288ca63@ozlabs.ru> (raw)
In-Reply-To: <20200908054416.GA13585@lst.de>
On 08/09/2020 15:44, Christoph Hellwig wrote:
> On Tue, Sep 08, 2020 at 11:51:06AM +1000, Alexey Kardashevskiy wrote:
>> What is dma_get_required_mask() for anyway? What "requires" what here?
>
> Yes, it is a really odd API. It comes from classic old PCI where
> 64-bit addressing required an additional bus cycle, and various devices
> had different addressing schemes, with the smaller addresses beeing
> more efficient. So this allows the driver to request the "required"
> addressing mode to address all memory. "preferred" might be a better
> name as we'll bounce buffer if it isn't met. I also don't really see
> why a driver would ever want to use it for a modern PCIe device.
a-ha, this makes more sense, thanks. Then I guess we need to revert that
one bit from yours f1565c24b596, do not we?
--
Alexey
next prev parent reply other threads:[~2020-09-08 12:09 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-08 1:51 [PATCH kernel] powerpc/dma: Fix dma_map_ops::get_required_mask Alexey Kardashevskiy
2020-09-08 5:44 ` Christoph Hellwig
2020-09-08 12:06 ` Alexey Kardashevskiy [this message]
2020-09-08 12:19 ` Christoph Hellwig
[not found] ` <1746dd66810.27bb.1ca38dd7e845b990cd13d431eb58563d@ozlabs.ru>
[not found] ` <20200909075849.GA12282@lst.de>
2020-09-09 9:36 ` Alexey Kardashevskiy
2020-09-15 6:50 ` Christoph Hellwig
2020-09-22 2:26 ` Alexey Kardashevskiy
2020-09-23 14:10 ` Christoph Hellwig
2020-09-24 7:03 ` Alexey Kardashevskiy
2020-09-25 4:56 ` Christoph Hellwig
2021-09-29 8:48 ` Alexey Kardashevskiy
2020-09-08 6:45 ` Michael Ellerman
2020-09-08 11:45 ` Cédric Le Goater
2020-09-10 12:55 ` Michael Ellerman
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