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Thu, 16 Apr 2026 13:21:07 -0700 (PDT) Received: from redhat.com ([2600:382:8128:ce33:b0f6:e3dc:ef2e:339b]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-50e1ad963ccsm55132891cf.2.2026.04.16.13.21.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Apr 2026 13:21:07 -0700 (PDT) Date: Thu, 16 Apr 2026 16:21:04 -0400 From: Brian Masney To: Prabhakar Cc: Michael Turquette , Stephen Boyd , Geert Uytterhoeven , linux-kernel@vger.kernel.org, linux-clk@vger.kernel.org, linux-renesas-soc@vger.kernel.org, Biju Das , Fabrizio Castro , Lad Prabhakar Subject: Re: [PATCH v2 1/2] clk: divider: Fix clk_divider_bestdiv() returning min rate for large rate requests Message-ID: References: <20260413124912.3260571-1-prabhakar.mahadev-lad.rj@bp.renesas.com> <20260413124912.3260571-2-prabhakar.mahadev-lad.rj@bp.renesas.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260413124912.3260571-2-prabhakar.mahadev-lad.rj@bp.renesas.com> User-Agent: Mutt/2.3.1 (2026-03-20) On Mon, Apr 13, 2026 at 01:49:11PM +0100, Prabhakar wrote: > From: Lad Prabhakar > > clk_divider_bestdiv() clamps maxdiv using: > > maxdiv = min(ULONG_MAX / rate, maxdiv); > > to avoid overflow in rate * i. However, requests like > clk_round_rate(clk, ULONG_MAX), which are used to determine the maximum > supported rate of a clock, result in maxdiv being clamped to 1. If no > valid divider of 1 exists in the table the loop is never entered and > bestdiv falls back to the maximum divider with the minimum parent rate, > causing clk_round_rate(clk, ULONG_MAX) to incorrectly return the minimum > supported rate instead of the maximum. > > Fix this by removing the pre-loop maxdiv clamping and replacing the > unprotected rate * i multiplication with check_mul_overflow(). Guard > the exact-match short-circuit with !overflow to prevent a clamped > target_parent_rate of ULONG_MAX from falsely matching parent_rate_saved > and causing premature loop exit. Break out of the loop after evaluating > the first overflowing divider since clk_hw_round_rate(parent, ULONG_MAX) > returns a constant for all subsequent iterations, meaning no better > candidate can be found, and continuing would cause exponential recursive > calls in chained divider clocks. > > Signed-off-by: Lad Prabhakar Reviewed-by: Brian Masney