linux-kernel - [PATCH 8/9] IB/qib: Sign extend without triggering implementation-defined behavior

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Date:   Mon, 28 Oct 2019 13:06:59 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        Bart Van Assche <bvanassche@....org>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Doug Ledford <dledford@...hat.com>
Subject: [PATCH 8/9] IB/qib: Sign extend without triggering implementation-defined behavior

>From the C standard: "The result of E1 >> E2 is E1 right-shifted E2 bit
positions. If E1 has an unsigned type or if E1 has a signed type and a
nonnegative value, the value of the result is the integral part of the
quotient of E1 / 2E2 . If E1 has a signed type and a negative value, the
resulting value is implementation-defined."

Hence use sign_extend_24_to_32() instead of "<< 8 >> 8".

Cc: Dennis Dalessandro <dennis.dalessandro@...el.com>
Cc: Mike Marciniszyn <mike.marciniszyn@...el.com>
Cc: Jason Gunthorpe <jgg@...lanox.com>
Cc: Doug Ledford <dledford@...hat.com>
Signed-off-by: Bart Van Assche <bvanassche@....org>
---
 drivers/infiniband/hw/qib/qib_rc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/infiniband/hw/qib/qib_rc.c b/drivers/infiniband/hw/qib/qib_rc.c
index aaf7438258fa..2f1beaab6935 100644
--- a/drivers/infiniband/hw/qib/qib_rc.c
+++ b/drivers/infiniband/hw/qib/qib_rc.c
@@ -566,7 +566,7 @@ int qib_make_rc_req(struct rvt_qp *qp, unsigned long *flags)
 		break;
 	}
 	qp->s_sending_hpsn = bth2;
-	delta = (((int) bth2 - (int) wqe->psn) << 8) >> 8;
+	delta = sign_extend_24_to_32(bth2 - wqe->psn);
 	if (delta && delta % QIB_PSN_CREDIT == 0)
 		bth2 |= IB_BTH_REQ_ACK;
 	if (qp->s_flags & RVT_S_SEND_ONE) {
-- 
2.24.0.rc0.303.g954a862665-goog