RE: returned dma address value of dma_alloc_coherent, is it always in the lower 4GB range?

From: "Chan Kim" <ckim@etri.re.kr>
To: "'Valdis Klētnieks'" <valdis.kletnieks@vt.edu>
Cc: kernelnewbies@kernelnewbies.org
Subject: RE: returned dma address value of dma_alloc_coherent, is it always in the lower 4GB range?
Date: Tue, 14 Mar 2023 17:03:28 +0900	[thread overview]
Message-ID: <043801d9564b$766e5b20$634b1160$@etri.re.kr> (raw)
In-Reply-To: <687742.1678770890@turing-police>

Hello Valdis,

>-----Original Message-----
>From: Valdis Kletnieks <valdis@vt.edu> On Behalf Of Valdis Kl?tnieks
>Sent: Tuesday, March 14, 2023 2:15 PM
>To: Chan Kim <ckim@etri.re.kr>
>Cc: kernelnewbies@kernelnewbies.org
>Subject: Re: returned dma address value of dma_alloc_coherent, is it always
>in the lower 4GB range?
>
>On Mon, 13 Mar 2023 21:28:28 +0900, "Chan Kim" said:
>
>> which is 32-bit addressable.  Even if the device indicates (via the
>> DMA
>> mask)
>> that it may address the upper 32-bits, consistent allocation will only
>> return > 32-bit addresses for DMA if the consistent DMA mask has been
>> explicitly changed via dma_set_coherent_mask().  This is true of the
>> dma_pool interface as well.
>> ------------------------------
>
>> By context I guess it returns the address in lower 32bit address range
>> even it the coherent_dma_mask is set to some bigger value.
>
>No, you have that backwards.   It's not "even if".  It wil give you a
32-bit
>address *UNLESS* the driver has called dma_set_coherent_mask() to set a
>wider mask *and* the device indicates it supports more than 32 bit
>addresses...

I'm sorry but please allow me to ask one more time.
1. "The consistent DMA mapping interfaces, will by default return a DMA
address
which is 32-bit addressable."  
==> so by default consistent dma mapping API returns 32-bit address, and
this 32-bit address means dma address, not physical address. Am I correct?
(because the dma address will be translated to physical address by the iommu
anyway)

2. " Even if the device indicates (via the DMA mask)
that it may address the upper 32-bits, consistent allocation will only
return > 32-bit addresses for DMA if the consistent DMA mask has been
explicitly changed via dma_set_coherent_mask().  This is true of the
dma_pool interface as well." 
==> So I now understand the first 'even if' is for streaming dma mask
('dma_mask' in the struct device), and the second 'if' is for the
'coherent_dma_mask' in the struct device. And when coherent_dma_mask is not
set, by default, the coherent mapping returns 32 bit address, but if the
coherent_dma_mask is set to use wider than 32 bit addresses, it returns the
wider address which seems obvious now.

Please correct me if I'm wrong.
Thank you!

Chan Kim

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