FROMLIST: misc: fastrpc: Replace hardcoded ctxid mask with GENMASK

Replace the hardcoded context ID mask (0xFF0) with GENMASK(11, 4) to
improve readability and follow kernel bitfield conventions. Use
FIELD_PREP and FIELD_GET instead of manual shifts for setting and
extracting ctxid values.

Link: https://lore.kernel.org/all/20260215182136.3995111-3-ekansh.gupta@oss.qualcomm.com/
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>

Author: ekanshibu

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 GENMASK(11, 4)
 #define INIT_FILELEN_MAX (2 * 1024 * 1024)
 #define INIT_FILE_NAMELEN_MAX (128)
 #define FASTRPC_DEVICE_NAME	"fastrpc"
@@ -515,7 +515,7 @@ 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, FIELD_GET(FASTRPC_CTXID_MASK, ctx->ctxid));
 	spin_unlock_irqrestore(&cctx->lock, flags);
 
 	kfree(ctx->maps);
@@ -649,7 +649,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
 		spin_unlock_irqrestore(&cctx->lock, flags);
 		goto err_idr;
 	}
-	ctx->ctxid = ret << 4;
+	ctx->ctxid = FIELD_PREP(FASTRPC_CTXID_MASK, ret);
 	spin_unlock_irqrestore(&cctx->lock, flags);
 
 	kref_init(&ctx->refcount);
@@ -2504,7 +2504,7 @@ 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 = FIELD_GET(FASTRPC_CTXID_MASK, rsp->ctx);
 
 	spin_lock_irqsave(&cctx->lock, flags);
 	ctx = idr_find(&cctx->ctx_idr, ctxid);