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

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Message-ID: <rszs7ttr6eibkzwfykrtokua37lxpeph6lq5t5c2rwemahghcr@zicxvouknr2n>
Date: Mon, 8 Sep 2025 12:41:07 +0300
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 v1 2/3] misc: fastrpc: Update context ID mask for polling
 mode support

On Mon, Sep 08, 2025 at 10:42:10AM +0530, Ekansh Gupta wrote:
> 
> 
> 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.

This sounds like a perfect part of the commit message.

> >
> >> 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
> >>
> 

-- 
With best wishes
Dmitry