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Fri, 18 Sep 2026 04:23:23 -0500 Message-ID: <307c005e-9a37-41cc-871a-6167a54d3a37@ti.com> Date: Fri, 18 Sep 2026 14:53:22 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] serial: 8250_dw: Add capability to skip empty FIFO read To: =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= , "Kumar, Udit" CC: Greg Kroah-Hartman , , linux-serial , LKML , Jiri Slaby , , , , , , References: <20260916085520.2259420-1-m-shah@ti.com> <20260916085520.2259420-3-m-shah@ti.com> <5fe0efdf-6d61-dba4-9703-269dfd68ae13@linux.intel.com> <5801b899-8090-4bff-b4e2-c3e8c97b0eb3@ti.com> Content-Language: en-US From: Moteen Shah In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS3PEPF0000C37E:EE_|CY5PR10MB5962:EE_ X-MS-Office365-Filtering-Correlation-Id: e3c5b653-3efc-403a-762b-08df1566912d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|36860700016|23010399003|1800799024|376014|10067099003|4143699003|6133799003|18002099003|22082099003|56012099006|5023799004; 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The UART core also performs >>>> unconditional reads on the empty FIFO during startup and shutdown >>>> of the port. On the IP version used in TDA54, that interrupt storm >>>> no longer occurs, but reading an empty FIFO instead triggers a data >>>> abort. >>>> >>>> Add a new capability to guard against the empty FIFO reads, avoiding >>>> the data aborts. >>>> >>>> Signed-off-by: Moteen Shah >>>> --- >>>> drivers/tty/serial/8250/8250.h | 1 + >>>> drivers/tty/serial/8250/8250_dw.c | 16 +++++++++++++++- >>>> drivers/tty/serial/8250/8250_port.c | 12 +++++++++--- >>>> 3 files changed, 25 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h >>>> index 77fe0588fd6b..45e13c3a8c14 100644 >>>> --- a/drivers/tty/serial/8250/8250.h >>>> +++ b/drivers/tty/serial/8250/8250.h >>>> @@ -86,6 +86,7 @@ struct serial8250_config { >>>> * STOP PARITY EPAR SPAR WLEN5 WLEN6 >>>> */ >>>> #define UART_CAP_NOTEMT BIT(18) /* UART without interrupt on TEMT available */ >>>> +#define UART_CAP_RXFIFO_EMPTY_READ BIT(19) /* UART needs LSR_DR check before RX read (TDA54) */ >>> IMO, this define naming contradicts with the comment because you >>> effectively say "capable of reading Rx while receive buffer is empty", not >>> that it needs DR check before issuing that read on buffer (~ named exactly >>> opposite of the actual meaning it is being used in the code). >>> >> I echo your comments; the naming should reflect that the IP does not >> allow reading an empty FIFO. > I now recalled we've also the pending RBR -> SRBR change: > > https://lore.kernel.org/linux-serial/20260908141524.20302-1-yang.yicong@picoheart.com/#t > > I wonder if a similar thing would help in this case as well? > > I understand this patch covers more ground than the SRBR patch but can the > IP used in this case read SRBR without aborting? The IP that TDA54 uses does not supports the shadow registers, hence we will have to go through the quirk/capability path. Regards, Moteen > >> At the same time, I am thinking, reading the FIFO based on the >> UART_LSR_DR bit being set in the serial8250_clear_interrupts and >> serial8250_do_shutdown functions. >> Do you see any side effects to this? > >