linux-kernel - Re: [PATCH v5 3/4] misc: fastrpc: Expand context ID mask for DSP polling mode support

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Message-ID: <37e2876f-6998-4e79-8696-c7f97307a28f@oss.qualcomm.com>
Date: Thu, 29 Jan 2026 15:56:55 +0530
From: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: srini@...nel.org, linux-arm-msm@...r.kernel.org,
        gregkh@...uxfoundation.org, quic_bkumar@...cinc.com,
        linux-kernel@...r.kernel.org, quic_chennak@...cinc.com,
        dri-devel@...ts.freedesktop.org, arnd@...db.de,
        konrad.dybcio@....qualcomm.com
Subject: Re: [PATCH v5 3/4] misc: fastrpc: Expand context ID mask for DSP
 polling mode support



On 1/6/2026 8:23 AM, Dmitry Baryshkov wrote:
> On Tue, Dec 30, 2025 at 11:58:30AM +0530, Ekansh Gupta wrote:
>> Current FastRPC context uses a 12-bit mask:
>>   [ID(8 bits)][PD type(4 bits)] = GENMASK(11, 4)
> Is it Linux-only representation or is it also used by the DSP? Will it
> work with MSM8916?
Apologies for the delay in response, had to go back to check on older DSP software.

DSP will also use this but it handled properly across platforms. DSP get PD details from PD bits
and mask it to use further for async call checks. Other than async call check, the context ID is
majorly used for book-keeping on DSP side(no functional utility).

So, it should also work on MSM8916,
>
>> This works for normal calls but fails for DSP polling mode.
>> Polling mode expects a 16-bit layout:
>>   [15:8] = context ID (8 bits)
>>   [7:5]  = reserved
>>   [4]    = async mode bit
>>   [3:0]  = PD type (4 bits)
>>
>> If async bit (bit 4) is set, DSP disables polling. With current
>> mask, odd IDs can set this bit, causing DSP to skip poll updates.
>>
>> Update FASTRPC_CTXID_MASK to GENMASK(15, 8) so IDs occupy upper
>> byte and lower byte is left for DSP flags and PD type.
>>
>> Reserved bits remain unused. This change is compatible with
>> polling mode and does not break non-polling behavior.
>>
>> Bit layout:
>>   [15:8] = CCCCCCCC (context ID)
>>   [7:5]  = xxx (reserved)
>>   [4]    = A (async mode)
>>   [3:0]  = PPPP (PD type)
>>
>> Signed-off-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
>> ---
>>  drivers/misc/fastrpc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>