linux-kernel - Re: [PATCH v2 4/4] clocksource/drivers/timer-mediatek: Make timer-mediatek become loadable module

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Date:   Wed, 15 Feb 2023 13:43:19 +0100
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Sudeep Holla <sudeep.holla@....com>, walter.chang@...iatek.com
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "Maciej W . Rozycki" <macro@...am.me.uk>,
        John Stultz <jstultz@...gle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        wsd_upstream@...iatek.com, stanley.chu@...iatek.com,
        Chun-hung.Wu@...iatek.com, Freddy.Hsin@...iatek.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v2 4/4] clocksource/drivers/timer-mediatek: Make
 timer-mediatek become loadable module

Il 14/02/23 23:20, Sudeep Holla ha scritto:
> On Tue, Feb 14, 2023 at 06:53:14PM +0800, walter.chang@...iatek.com wrote:
>> From: Chun-Hung Wu <chun-hung.wu@...iatek.com>
>>
>> Make the timer-mediatek driver which can register
>> an always-on timer as tick_broadcast_device on
>> MediaTek SoCs become loadable module in GKI.
>>
>> Signed-off-by: Chun-Hung Wu <chun-hung.wu@...iatek.com>
>> ---
>>   drivers/clocksource/Kconfig          |  2 +-
>>   drivers/clocksource/timer-mediatek.c | 43 ++++++++++++++++++++++++++++
>>   2 files changed, 44 insertions(+), 1 deletion(-)
> 
> [...]
> 
>> diff --git a/drivers/clocksource/timer-mediatek.c b/drivers/clocksource/timer-mediatek.c
>> index d5b29fd03ca2..3358758ea694 100644
>> --- a/drivers/clocksource/timer-mediatek.c
>> +++ b/drivers/clocksource/timer-mediatek.c
> 
> [...]
> 
>> +static const struct of_device_id mtk_timer_match_table[] = {
>> +	{
>> +		.compatible = "mediatek,mt6577-timer",
>> +		.data = mtk_gpt_init,
>> +	},
>> +	{
>> +		.compatible = "mediatek,mt6765-timer",
>> +		.data = mtk_syst_init,
>> +	},
>> +	{
>> +		.compatible = "mediatek,mt6795-systimer",
>> +		.data = mtk_cpux_init,
>> +	},
>> +	{}
>> +};
>> +
>> +static struct platform_driver mtk_timer_driver = {
>> +	.probe = mtk_timer_probe,
>> +	.driver = {
>> +		.name = "mtk-timer",
>> +		.of_match_table = mtk_timer_match_table,
>> +	},
>> +};
>> +module_platform_driver(mtk_timer_driver);
>> +
>> +MODULE_DESCRIPTION("MediaTek Module Timer driver");
>> +MODULE_LICENSE("GPL v2");
>> +#else
>>   TIMER_OF_DECLARE(mtk_mt6577, "mediatek,mt6577-timer", mtk_gpt_init);
>>   TIMER_OF_DECLARE(mtk_mt6765, "mediatek,mt6765-timer", mtk_syst_init);
>>   TIMER_OF_DECLARE(mtk_mt6795, "mediatek,mt6795-systimer", mtk_cpux_init);
> 
> Why do you need these ? If this driver can work as a module, it can be
> built-in module and doesn't need to be initialised early using of_timer_init
> (can't recall the exact name)
> 
> 

Some platforms need early initialization; this is seen on ones for which the
bootloader does not initialize the "CPUXGPT" timer, which is used as the ARM
arch timer. (No, on those platforms you can't upgrade the bootloader, as it's
signed with a OEM key which is not obtainable, and signature verified earlier
in the bootchain).

As a matter of fact (and somehow obvious), on those platforms (for example,
MT6795.. but many other as well, really), you *need* this driver to be
built-in and, well, initialize the CPUX timer as early as possible :-)

Regards,
Angelo