linux-kernel - Re: [PATCH 1/1] xilinx ps uart: Adding a kernel parameter for the number of xilinx ps uarts

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Date:   Tue, 23 May 2017 13:44:24 +0200
From:   Michal Simek <michal.simek@...inx.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Michal Simek <michal.simek@...inx.com>,
        Rob Herring <robh+dt@...nel.org>
CC:     Sam Povilus <kernel.development@...il.us>,
        <gregkh@...uxfoundation.org>, <jslaby@...e.com>,
        <soren.brinkmann@...inx.com>, <linux-serial@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] xilinx ps uart: Adding a kernel parameter for the
 number of xilinx ps uarts

On 22.5.2017 20:26, Alan Cox wrote:
>> We have in soc vendor tree similar patch but the reason is different.
>>
>>     tty: serial: Added a CONFIG_SERIAL_XILINX_NR_UARTS option.
>>
>>     This patch Adds CONFIG_SERIAL_XILINX_NR_UARTS option to allow
>>     the user to provide the Max number of uart ports information.
>>     If multiple cards (or) PL UARTS are present, the default limit
>>     of 2 ports should be increased.
>>
>> I haven't checked all drivers but in our case we have added this as
>> quick fix for scenarios where you use serial aliases where alias is
>> pointed to serial2 or more.
>> In cdns_uart_init() cdns_uart_uart_driver is passed which contains .nr
>> which is required to be passed.
>>
>> What's the best driver to look at dynamic allocation?
> 
> So there are quite a few that dynamically allocate the objects as they
> are enumerated (eg max3100), but have a maximum set that is just pointers
> (so for the max number of ports cheaper than the dynamic code)

yep hardcoded max 4 where in probe first free space is found and used
(range 0-3) but still max3100s statically allocated.
Shouldn't be this also dynamically allocated?


> The other question is why is it a CONFIG_ option. I'm assuming these
> platforms are all ARM and in that case you could just pass the value in
> the device tree, or hard code a safe maximum number of pointers to a
> value which is the worst case and then install them as they are
> enumerated.

I am not quite sure how exactly you want to do this via DT.

Also what do you think is a safe maximum number? This is fpga - hundreds
of pins which can do just uart.

> There are lots of options better than breaking the "one kernel many
> platforms" model.

Another options is also module parameter and dynamically allocated array
in cdns_uart_init.

Thanks,
Michal