From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sam Ravnborg Subject: Re: [PATCH v1 2/5] pardata: new bus for parallel data access Date: Wed, 8 Aug 2018 10:24:31 +0200 Message-ID: <20180808082431.GA29143@ravnborg.org> References: <20180802193909.GA11443@ravnborg.org> <20180802194536.10820-2-sam@ravnborg.org> <634ea7d9-7cc1-f404-b072-0bafbc748fb7@tronnes.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: <634ea7d9-7cc1-f404-b072-0bafbc748fb7@tronnes.org> Sender: linux-kernel-owner@vger.kernel.org To: Noralf =?iso-8859-1?Q?Tr=F8nnes?= Cc: Greg Kroah-Hartman , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org List-Id: dri-devel@lists.freedesktop.org Hi Noralf. On Tue, Aug 07, 2018 at 06:40:29PM +0200, Noralf Trønnes wrote: > Hi Sam, > > Den 02.08.2018 21.45, skrev Sam Ravnborg: > >The pardata supports implement a simple bus for devices > >that are connected using a parallel bus driven by GPIOs. > >The is often used in combination with simple displays > >that is often seen in older embedded designs. > >There is a demand for this support also in the linux > >kernel for HW designs that uses these kind of displays. > > > >The pardata bus uses a platfrom_driver that when probed > >creates devices for all child nodes in the DT, > >which are then supposed to be handled by pardata_drivers. > > > >Signed-off-by: Sam Ravnborg > >--- > > From a quick look at this I have these comments: > 1. There can only be one implementation of this bus, the gpio one. >    There are SOC's with parallel bus hardware so you need to allow for >    more implementations. > 2. The client shouldn't do the bus signaling. This should be hidden >    behind read and write functions in pardata. > 3. I would also suggest you add an address bus instead of the RS pin > 4. I don't think reset belongs in the bus. It's a device thing. Excellent feedback - I will address this in v2. > 5. You can use gpiod_set_array_value() in the gpio implementation. >    Some gpio drivers can set all gpios at once. Yes, it is already used. But only for DB0 to DB7 as there are timing constraings for the others. > > I made an attempt at implementing a bus like this a while back: > https://github.com/notro/fbdbi/tree/master/i80 Thanks, very helpfull. v2 will take a while as I plan to have something that actually works before posting next version. One open question. Miguel Ojeda mentioned that there is already a limited fbdev driver made on top of parport (part of auxdisplay). Is it the correct design to come up with a new bus or should this try to build on top of parport? I did not check in details if using parport is possible, but from a quick look it is doable. But then we use parport for something that it originally was not designed for and we drag with us a lot of extra functionality. So I like the slimmer pardata bus. Sam