linux-kernel - Re: [PATCH v3 2/3] misc: fastrpc: Update context ID mask for polling mode support

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Message-ID: <b3bb2b91-c938-4f7b-bbb7-52b18743b696@oss.qualcomm.com>
Date: Thu, 27 Nov 2025 12:21:49 +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
Subject: Re: [PATCH v3 2/3] misc: fastrpc: Update context ID mask for polling
 mode support



On 11/19/2025 1:59 PM, Dmitry Baryshkov wrote:
> On Thu, Nov 13, 2025 at 12:11:10PM +0530, Ekansh Gupta wrote:
>> Current FastRPC message context uses a 12-bit mask where the upper
>> 8 bits are context ID from idr_alloc_cyclic and the lower 4 bits
>> represent PD type. This design works for normal FastRPC calls but
>> doesn't work as expected for polling mode.
> Doesn't work why / how?
Explained this in the next statements, I'll rephrase this.
>
>> To enable polling mode
>> support from DSP(DSP writes to poll memory), DSP expects a 16-bit
>> context where the upper 8 bits are context ID, the lower 4 bits are
>> PD type and the 5th bit from the end denotes async mode(not yet
> What's in the middle between bits 5 and 8?
It's unused as of now.
>
>> upstreamed). If this bit is set, DSP disables polling. With the
>> current design, odd context IDs set this bit, causing DSP to skip
>> poll memory updates. Update the context mask to ensure a hole
>> which won't get populated, ensuring polling mode works as expected.
>> This is not a bug and the change is added to support polling mode.
>>
>> Signed-off-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
>> ---
>>  drivers/misc/fastrpc.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
>> index 679cd8997a00..ccba3b6dfdfa 100644
>> --- a/drivers/misc/fastrpc.c
>> +++ b/drivers/misc/fastrpc.c
>> @@ -37,7 +37,8 @@
>>  #define FASTRPC_CTX_MAX (256)
>>  #define FASTRPC_INIT_HANDLE	1
>>  #define FASTRPC_DSP_UTILITIES_HANDLE	2
>> -#define FASTRPC_CTXID_MASK (0xFF0)
>> +#define FASTRPC_CTXID_MASK (0xFF00)
>> +#define FASTRPC_CTXID_POS (8)
> Use FIELD_PREP instead.
Checking this, thanks
>
>>  #define INIT_FILELEN_MAX (2 * 1024 * 1024)
>>  #define INIT_FILE_NAMELEN_MAX (128)
>>  #define FASTRPC_DEVICE_NAME	"fastrpc"
>> @@ -489,7 +490,7 @@ static void fastrpc_context_free(struct kref *ref)
>>  		fastrpc_buf_free(ctx->buf);
>>  
>>  	spin_lock_irqsave(&cctx->lock, flags);
>> -	idr_remove(&cctx->ctx_idr, ctx->ctxid >> 4);
>> +	idr_remove(&cctx->ctx_idr, ctx->ctxid >> FASTRPC_CTXID_POS);
>>  	spin_unlock_irqrestore(&cctx->lock, flags);
>>  
>>  	kfree(ctx->maps);
>> @@ -625,7 +626,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
>>  		spin_unlock_irqrestore(&cctx->lock, flags);
>>  		goto err_idr;
>>  	}
>> -	ctx->ctxid = ret << 4;
>> +	ctx->ctxid = ret << FASTRPC_CTXID_POS;
>>  	spin_unlock_irqrestore(&cctx->lock, flags);
>>  
>>  	kref_init(&ctx->refcount);
>> @@ -2451,7 +2452,7 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
>>  	if (len < sizeof(*rsp))
>>  		return -EINVAL;
>>  
>> -	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> 4);
>> +	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> FASTRPC_CTXID_POS);
>>  
>>  	spin_lock_irqsave(&cctx->lock, flags);
>>  	ctx = idr_find(&cctx->ctx_idr, ctxid);
>> -- 
>> 2.34.1
>>