linux-kernel - Re: [PATCH 2/4] soc: apple: rtkit: Implement OSLog buffers properly

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Message-ID: <Z7y14Q3ifu7U1tHI@blossom>
Date: Mon, 24 Feb 2025 13:09:37 -0500
From: Alyssa Rosenzweig <alyssa@...enzweig.io>
To: sven@...npeter.dev
Cc: Janne Grunau <j@...nau.net>, asahi@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Hector Martin <marcan@...can.st>
Subject: Re: [PATCH 2/4] soc: apple: rtkit: Implement OSLog buffers properly

> +	if (ep == APPLE_RTKIT_EP_OSLOG) {
> +		buffer->size = FIELD_GET(APPLE_RTKIT_OSLOG_SIZE, msg);
> +		buffer->iova = FIELD_GET(APPLE_RTKIT_OSLOG_IOVA, msg) << 12;
> +	} else {
> +		buffer->size = FIELD_GET(APPLE_RTKIT_BUFFER_REQUEST_SIZE, msg) << 12;
> +		buffer->iova = FIELD_GET(APPLE_RTKIT_BUFFER_REQUEST_IOVA, msg);
> +	}

The shifts are suspiciously asymmetric. Are we really sure this is
correct? My guess is that both size & iova for both oslog & buffer need
to be page-aligned, so all 4 lines should be shifted, and the bit
offsets should be adjusted in turn, and the lower 12-bits in oslog_size
and buffer_iova are reserved. But that's just a guess.

Anyway if this logic is really what we want it deserves a comment
because it looks like a typo.

(Likewise later in the patch)