linux-kernel - Re: [PATCH v1 2/3] misc: fastrpc: Update context ID mask for polling mode support

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Message-ID: <7250915d-4611-4f81-a46d-b85e76185aae@oss.qualcomm.com>
Date: Mon, 8 Sep 2025 10:42:10 +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 v1 2/3] misc: fastrpc: Update context ID mask for polling
 mode support



On 9/2/2025 9:42 AM, Dmitry Baryshkov wrote:
> On Mon, Sep 01, 2025 at 11:03:35AM +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
> FastRPC (here and in several other places).
Ack.
>
>> doesn't work as expected for polling mode. 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
>> 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.
> This looks like a description of a bugfix. In such a case, move it to
> the start of the series, add necessary tatgs and change commit message
> accordingly.
I'm not sure if it could be called a bug fix as there is no problem with existing mask.
The problem is seen only when polling mode is added.
>
>> Update the context mask to ensure a hole
>> which won't get populated, ensuring polling mode works as expected.
>>
>> Signed-off-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
>> ---
>>  drivers/misc/fastrpc.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
>> index 0991ac487192..57e118de6e4a 100644
>> --- a/drivers/misc/fastrpc.c
>> +++ b/drivers/misc/fastrpc.c
>> @@ -37,7 +37,7 @@
>>  #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 INIT_FILELEN_MAX (2 * 1024 * 1024)
>>  #define INIT_FILE_NAMELEN_MAX (128)
>>  #define FASTRPC_DEVICE_NAME	"fastrpc"
>> @@ -487,7 +487,7 @@ 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 >> 8);
> #define magic value
Ack.
>
>>  	spin_unlock_irqrestore(&cctx->lock, flags);
>>  
>>  	kfree(ctx->maps);
>> @@ -623,7 +623,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
>>  		spin_unlock_irqrestore(&cctx->lock, flags);
>>  		goto err_idr;
>>  	}
>> -	ctx->ctxid = ret << 4;
>> +	ctx->ctxid = ret << 8;
>>  	spin_unlock_irqrestore(&cctx->lock, flags);
>>  
>>  	kref_init(&ctx->refcount);
>> @@ -2449,7 +2449,7 @@ 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) >> 8);
>>  
>>  	spin_lock_irqsave(&cctx->lock, flags);
>>  	ctx = idr_find(&cctx->ctx_idr, ctxid);
>> -- 
>> 2.34.1
>>