linux-kernel - Re: [PATCH 1/2] misc: fastrpc: Add support for new DSP IOVA formatting

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Message-ID: <37ac6311-bd88-4b8e-bf78-06fcdd8bdf6f@oss.qualcomm.com>
Date: Tue, 30 Sep 2025 10:10:32 +0530
From: Kumari Pallavi <kumari.pallavi@....qualcomm.com>
To: Arnd Bergmann <arnd@...db.de>, Jingyi Wang
 <jingyi.wang@....qualcomm.com>,
        Srinivas Kandagatla <srini@...nel.org>,
        Amol Maheshwari <amahesh@....qualcomm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: aiqun.yu@....qualcomm.com, tingwei.zhang@....qualcomm.com,
        trilok.soni@....qualcomm.com, yijie.yang@....qualcomm.com,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] misc: fastrpc: Add support for new DSP IOVA
 formatting



On 9/25/2025 11:54 AM, Arnd Bergmann wrote:
> On Thu, Sep 25, 2025, at 01:46, Jingyi Wang wrote:
> 
>>   	dma_free_coherent(buf->dev, buf->size, buf->virt,
>> -			  FASTRPC_PHYS(buf->phys));
>> +			  IOVA_TO_PHYS(buf->phys, sid_pos));
>>   	kfree(buf);
>>   }
> 
> I understand what you are doing, but the naming of the macros
> seems a bit confusing: dma_free_coherent() and the related
> functions are designed to take an IOVA argument, not a physical
> address, so calling IOVA_TO_PHYS() before passing the
> address sounds wrong. This is made worse by the naming
> of 'buf->phys' that is not a physical address at all
> but already transformed twice into a dma_addr_t and
> from there into a dma_addr+sid tuple.
> 
> Ideally the SID handling would be abstracted behind a custom
> dma_map_ops implementation that treats this as a custom
> iommu, but if the fastrpc device is the only user of this
> address format, I can understand you want to keep this as
> a local hack in this driver.
> 
> Can you try to come up with some better naming here, and
> replace the 'phys' bits with something that is more fitting
> for an iova/dma_addr_t?
>

I will improve the naming for better clarity:

- Replace buf->phys with buf->dma_addr throughout the code.
- Rename the macro IOVA_TO_PHYS() to something more appropriate, like 
IPA_TO_DMA_ADDR()
>       Arnd

Thanks,
Pallavi