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

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Message-ID: <jjzyh65ym74si2u7ig5gtckffma522ebjbeqyr4ym2sddox6iu@l7uion5z2rvf>
Date: Wed, 19 Nov 2025 10:29:22 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Ekansh Gupta <ekansh.gupta@....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 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?

> 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?

> 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.

>  #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
> 

-- 
With best wishes
Dmitry