From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 71D7CC433DB for ; Wed, 20 Jan 2021 17:10:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 241B3233EB for ; Wed, 20 Jan 2021 17:10:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733093AbhATRKI (ORCPT ); Wed, 20 Jan 2021 12:10:08 -0500 Received: from mail.kernel.org ([198.145.29.99]:58832 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2391751AbhATRF7 (ORCPT ); Wed, 20 Jan 2021 12:05:59 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id D8BCD2137B; Wed, 20 Jan 2021 17:05:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1611162310; bh=WtZFfFv8GO9q/O1gGxEeXgX0Wj/VD7wZE2sc6QJ27o8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=tPkkOWw/X0F+3FvGsBJreKu0ARlTqj2ae5kmYBGEHJsL0wFLXH/ry8cs1XUhspE1K XWE2Af1UCMcHpVyt6q9crwhI3ePHzZ0Vj4bCUAxRaDHQSvUsEnbm4OoyEt3WbhSM1z N0IxR5YOojYYYqiuWotYnZSTFPYMI3RK75JLodGk= Date: Wed, 20 Jan 2021 18:05:07 +0100 From: Greg Kroah-Hartman To: Andy Shevchenko Cc: Florian Fainelli , Al Cooper , Linux Kernel Mailing List , bcm-kernel-feedback-list , devicetree , Jiri Slaby , "open list:SERIAL DRIVERS" , USB , Masahiro Yamada , Rob Herring Subject: Re: [PATCH v2 2/2] serial: 8250: Add new 8250-core based Broadcom STB driver Message-ID: References: <20210115211543.33563-1-alcooperx@gmail.com> <20210115211543.33563-3-alcooperx@gmail.com> <71d58a3e-2707-69d7-8074-c67235912e06@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 20, 2021 at 06:47:52PM +0200, Andy Shevchenko wrote: > On Tue, Jan 19, 2021 at 8:16 PM Florian Fainelli wrote: > > On 1/19/2021 7:21 AM, Andy Shevchenko wrote: > > > On Fri, Jan 15, 2021 at 11:19 PM Al Cooper wrote: > > >> > > >> Add a UART driver for the new Broadcom 8250 based STB UART. The new > > >> UART is backward compatible with the standard 8250, but has some > > >> additional features. The new features include a high accuracy baud > > >> rate clock system and DMA support. > > >> > > >> The driver will use the new optional BAUD MUX clock to select the best > > >> one of the four master clocks (81MHz, 108MHz, 64MHz and 48MHz) to feed > > >> the baud rate selection logic for any requested baud rate. This allows > > >> for more accurate BAUD rates when high speed baud rates are selected. > > >> > > >> The driver will use the new UART DMA hardware if the UART DMA registers > > >> are specified in Device Tree "reg" property. The DMA functionality can > > >> be disabled on kernel boot with the argument: > > >> "8250_bcm7271.disable_dma=Y". > > >> > > >> The driver also set the UPSTAT_AUTOCTS flag when hardware flow control > > >> is enabled. This flag is needed for UARTs that don't assert a CTS > > >> changed interrupt when CTS changes and AFE (Hardware Flow Control) is > > >> enabled. > > >> > > >> The driver also contains a workaround for a bug in the Synopsis 8250 > > >> core. The problem is that at high baud rates, the RX partial FIFO > > >> timeout interrupt can occur but there is no RX data (DR not set in > > >> the LSR register). In this case the driver will not read the Receive > > >> Buffer Register, which clears the interrupt, and the system will get > > >> continuous UART interrupts until the next RX character arrives. The > > >> fix originally suggested by Synopsis was to read the Receive Buffer > > >> Register and discard the character when the DR bit in the LSR was > > >> not set, to clear the interrupt. The problem was that occasionally > > >> a character would arrive just after the DR bit check and a valid > > >> character would be discarded. The fix that was added will clear > > >> receive interrupts to stop the interrupt, deassert RTS to insure > > >> that no new data can arrive, wait for 1.5 character times for the > > >> sender to react to RTS and then check for data and either do a dummy > > >> read or a valid read. Sysfs error counters were also added and were > > >> used to help create test software that would cause the error condition. > > >> The counters can be found at: > > >> /sys/devices/platform/rdb/*serial/rx_bad_timeout_late_char > > >> /sys/devices/platform/rdb/*serial/rx_bad_timeout_no_char > > > > > > Brief looking into the code raises several questions: > > > - is it driver from the last decade? > > > > Work on this driver started back in 2018, that was indeed the last decade. > > > > > - why it's not using what kernel provides? > > > - we have a lot of nice helpers: > > > - DMA Engine API > > > > Not sure this makes sense, given that the DMA hardware that was added to > > this UART block is only used by the UART block and no other pieces of HW > > in the system, nor will they ever be. Not sure it makes sense to pay the > > cost of an extra indirection and subsystem unless there are at least two > > consumers of that DMA hardware to warrant modeling it after a dmaengine > > driver. I also remember that Al researched before whether 8250_dma.c > > could work, and came to the conclusion that it would not, but I will let > > him comment on the specifics. > > I see. In any case I still believe that the driver can be shrinked by > a notable amount of lines. Patches always gladly accepted :)