linux-kernel - Re: [PATCH v7 4/4] tty: serial: 8250: add DFL bus driver for Altera 16550.

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Message-ID: <d34b021c-eec4-905e-f352-734db2d8338a@redhat.com>
Date:   Wed, 21 Dec 2022 18:26:47 +0100
From:   Marco Pagani <marpagan@...hat.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        matthew.gerlach@...ux.intel.com
Cc:     hao.wu@...el.com, yilun.xu@...el.com, russell.h.weight@...el.com,
        basheer.ahmed.muddebihal@...el.com, trix@...hat.com,
        mdf@...nel.org, linux-fpga@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        tianfei.zhang@...el.com, corbet@....net,
        gregkh@...uxfoundation.org, linux-serial@...r.kernel.org,
        jirislaby@...nel.org, geert+renesas@...der.be,
        niklas.soderlund+renesas@...natech.se, macro@...am.me.uk,
        johan@...nel.org, lukas@...ner.de, ilpo.jarvinen@...ux.intel.com,
        bagasdotme@...il.com
Subject: Re: [PATCH v7 4/4] tty: serial: 8250: add DFL bus driver for Altera
 16550.



On 2022-12-20 18:09, Andy Shevchenko wrote:
> On Tue, Dec 20, 2022 at 08:36:52AM -0800, matthew.gerlach@...ux.intel.com wrote:
>> From: Matthew Gerlach <matthew.gerlach@...ux.intel.com>
>>
>> Add a Device Feature List (DFL) bus driver for the Altera
>> 16550 implementation of UART.
> 
> In general the code here looks good to me, but one thing to discuss due to
> comment to the previous patch(es).
> 
> ...
> 
>> +	u64 *p;
>> +
>> +	p = dfh_find_param(dfl_dev, DFHv1_PARAM_ID_CLK_FRQ);
>> +	if (!p)
>> +		return dev_err_probe(dev, -EINVAL, "missing CLK_FRQ param\n");
>> +
>> +	p++;
>> +	uart->port.uartclk = *p;
> 
> So, here and the below is using always the second u64 from the returned data.
> Does it mean:
> - we always skip the first u64 from the returned buffer and hence... (see below)
> - we may actually return the second u64 as a plain number (not a pointer) from
>   (an additional?) API? In such case we would not need to take care about this
>   p++; lines here and there.
> - we have fixed length of the data, returned by find_param(), i.e. 2 u64 words?
> 

I also had the impression that this method of getting and incrementing a pointer
to the beginning of the parameter block is a bit more error-prone than necessary.
Since parameter blocks are now standardized, wouldn't be easier and safer to wrap
the access logic into a helper function like:

u16 dfh_get_param_data(struct dfl_device *dfl_dev, u16 param_id, u64 *data)

that directly provides a copy of the parameter's data into a pointer provided by
the caller and returns the parameter version or an error if not found?

Thanks,
Marco