linux-kernel - Re: [PATCH v3 6/6] bus: mhi: host: pci_generic: Support independent DMA mask for VFs

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Message-ID: <3e827dd4-7e32-4d05-b113-87bb5175431c@quicinc.com>
Date: Fri, 12 Sep 2025 18:06:18 +0530
From: Vivek Pernamitta <quic_vpernami@...cinc.com>
To: Manivannan Sadhasivam <mani@...nel.org>, <Vivek.Pernamitta@...cinc.com>
CC: <mhi@...ts.linux.dev>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 6/6] bus: mhi: host: pci_generic: Support independent
 DMA mask for VFs



On 9/5/2025 8:33 PM, Manivannan Sadhasivam wrote:
> On Thu, Aug 21, 2025 at 06:25:38PM GMT, Vivek.Pernamitta@...cinc.com wrote:
>> From: Vivek Pernamitta <quic_vpernami@...cinc.com>
>>
>> Certain devices like QDU100 bootloader support only up to a 32-bit DMA
>> address range. However, Virtual Functions (VFs) are enabled only after
>> the device enters Mission Mode and can support higher DMA address ranges
>> (up to 40 bits).
>>
>> A 32-bit DMA mask limits addressable space to 4GiB, which is insufficient
>> for data transfer requirements over VFs on platforms like QDU100. These
>> devices require larger memory regions to be mapped for efficient VF
>> operation.
>>
>> To address this, configure `dma_mask` independently for Physical Functions
>> (PFs) and Virtual Functions (VFs), allowing VFs to use higher DMA mask
>> values where supported.
>>
>> As per PCIe SR-IOV specification (rev 0.9, Section 1), VFs are capable of
>> handling resources associated with the main data movement of the Function.
>>>> This change ensures compatibility with bootloaders that have limited DMA
>> capabilities while enabling full VF functionality once the device reaches
>> Mission Mode.
> 
> Compatibility is with the existing non-SR-IOV devices, isn't it? How does
> bootloader comes into picture for them?
> 
Set "vf_dma_data_width" explicitly for SR-IOV devices where VFs support
64-bit DMA but PFs do not. For non-SRIOV devices, this flag should 
remain dma_data_width unset and defaults to zero.  If both bootloader
and HLOS support the  same DMA width and for non-SRIOV device, set the
flag "dma_data_width" alone to the maximum supported bits to ensure
compatibility and performance.> - Mani
>